For some reason, there are not a lot of resources (or at least not that many that I've found) that describe a general approach to getting a good modern country guitar sound. Of course, now that I've said that I'm sure I'll get a dozen people pointing me sites that I somehow missed. But anyway, I'd like to go over what I've learned from various readings, speaking to other guitarists, and from my own experimentation.
The first thing that you notice when you listen to guitarists like Brad Paisley, Vince Gill, Jimmy Olander of Diamond Rio, and others is how twangy and compressed their sound is. Now, if you aren't familiar with what a compressor does and what it sounds like, then saying that a guitar sounds "compressed" doesn't mean anything to you. A compressor limits the peaks and boosts the valleys of a guitar's signal. That is, it makes the quiet sounds louder and the louder sounds quieter, which evens out the attack. It also has the effect of making the sound very poppy and punchy, if that makes sense. I'll get back to achieving this sound a little later.
For the twang part of the sound sound, that typically means "more treble" in your sound. Make sure that your tone knob is maxed and use your bridge pickup on whatever guitar you're using. You can boost this treble sound by tweaking the equalization on your amp to favor the treble side. That is, start with your Bass, Middle, and Treble knobs "flat" or right in the middle. If your knobs go from 1 to 10, put them at 5 or 6. Now, to further boost the treble you can raise the Treble knob and set it at 8 or so. Also, if your amp has a Bright switch, flip that on. If your amp has a Presence knob, which is like the next step above Treble, you can turn that up a bit to your taste.
So far we have the very basics of a great country tone. Let's take it to the next level and discuss the various pieces of gear and how you can use them to shape your sound.
Most country tricksters, as Johnny Hiland calls them, play Fender Telecaster which are known for their very twangy single-coil bridge pickups. You don't have to play a Telecaster to get a country sound, but using a guitar with a single-coil bridge pickup or a coil-splitting humbucker (which allows you to use the humbucker as a single-coil) definitely helps. Single-coil pickups have a much tighter, punchier sound that works well for country. That's not to say that you can't get a great country sound with a humbucker, but in general, most country guitarists play on Telecasters because of their bridge pickup.
Another common element that you might see on a guitar used by a country player is a maple fretboard, as opposed to a rosewood fretboard. There is some consensus that maple fretboards lead to a brighter sound. Not everyone agrees, but I've seen this to be true. However, and this is a BIG however, the difference is very subtle, in my opinion. I greatly prefer the feel of a rosewood fretboard so the extremely slight tone difference I found from a maple fretboard was definitely not worth it for me. Some people may feel that the tone difference is more noticeable, but after comparing the same American Telecasters with maple and rosewood fretboards, I could only barely tell the difference in tone but I could tell a great difference in feel and I prefer rosewood.
As for strings, I recommend a medium-gauge string, such as 9s, 10s, or 11s. For me, 9s are too light and 11s are a tad too heavy. You want a string that you can really bend on for those pedal steel double-stop oblique bends that are one of the signatures of country guitar. 10s have just the right amount of substance and tone while still allowing me to be flexible and bend as far as I need to.
If you're reading this, what I'm about to mention probably won't apply since it's a much more advanced feature, but I'll cover it anyway for reference. Some country guitarists have their guitars modified to include a B-bender or G-bender or even both. Until you see one, it's pretty difficult to understand exactly what it is, but I'll do my best to explain it.
Take the B-bender modification: from the front, the guitar looks pretty much unchanged except for a small ring behind bridge by the B string. The ball of the string is actually anchored in this ring rather than in the bridge, so the string goes past the bridge. Inside the guitar, there is a lever that is somehow (I've never actually seen the inside) connected to another lever by the neck-side strap post, which is spring-loaded. The shoulder strap then attaches to this new post and when you push the neck of the guitar down, the strap pulls against your shoulder and moves the lever which in turn pulls the lever by the ring and pulls the B-string tighter making the string ring sharp. It's almost like a whammy bar for a single string that is activated by pulling against the strap, or rather pushing the guitar down. This allows for pedal steel-style bends while playing. A G-bender is the same thing, except that it's typically pulling against the bridge-side strap post since most G-benders are implemented in addition to the B-bender. So with both, if you pushed down on the guitar on the neck and body, you'd get both the B and G strings to be pulled tighter, raising their respective pitches.
If that description was tough to follow, try taking a look at the pictures on this link: http://www.stringbender.com/bender/fenderb.html
Again, the B-bender and G-bender modification is not something that you will most likely need to worry about for now, but if you're completely befuddled by Jimmy Olander's licks, remember that he typically has a B and G-bender equipped guitar.
In order to further shape your sound, you're going to want employ a few effects pedals. Many of these can be achieved by pedals, rack effects, studio processing, or digital modelers, but I'm going to stick to pedals for this article.
First off, any good pedalboard should have a tuner pedal. It doesn't seem that great until you have one and you realize how handy it is. That applies to any style of guitar; you need a pedal tuner. I recommend the Boss TU-2 Chromatic Tuner.
Second, one of the key elements in modern country guitar is the compressor. There are many compressors out there and they vary in price range. I have a Boss CS-3 Compression Sustainer which has worked well for me. Some people have complaints about it, but they seem to be pretty standard for all compressors, not just the Boss. I've heard good things about Visual Sound Route 66 which has a compressor and an overdrive in one pedal. Guitarists seem to always be comparing the Ross Compressor (no, that's not a typo when I tried to type Boss, it is meant to be Ross) to all other pedals as if it was the mold that all pedals should made from. The MXR Dyna Comp also comes up quite often. Whichever compressor you choose, you want a highly "squished" sound. Try checking the manufacturer for suggestions for a "squished" or country sound and what settings achieve that sound. I typically run the Attack setting at about 60% and the Sustain at about 75% on my CS-3.
Third, you'll want some kind of delay or echo pedal to give you that quick slap-back sound that many country guitarists use. You don't want much delay at all. Something very subtle will do well as it should almost sound like reverb. I use an inexpensive Danelectro Fab Echo pedal for my slapback because it's cheap ($20) and it isn't meant for much delay, which is just what I want. I can also achieve this sound with my more expensive, fuller-featured delay pedals, but I'd rather save those for longer delays and echoes. As for settings, as I said, you want the delay time very short and you want it to repeat only once. When you play a single note, it should sound like a very quick and subtle "BUH-dup".
Lastly, I highly recommend a volume pedal for assisting with pedal steel volume swells. I use the Ernie Ball VP Jr., which works great for passive instruments (if your guitar doesn't require a 9v battery for EMG active pickups or something similar, then you have a passive guitar).
As for order, follow the standard effects ordering process that has been written about in a zillion posts and articles online. You'll also probably want a mild overdrive pedal and a chorus pedal, but these, like the tuner, are essential pedals to any setup, in my opinion.
This is where you will probably hear the widest variety of opinions in getting a country sound. My recommendation is either a Fender Deluxe Reverb or a Fender Twin Reverb. I play a Twin Reverb ('65 reissue), because I also like to use it for blues and it has a bright switch which the Deluxe Reverb is lacking. If you start paying attention to what your favorite guitarists play, you'll probably see Fender, Vox, Matchless, and Dr. Z (for Brad Paisley) amps. The common theme among all these amps is that they are all-tube combos with either one or two 12" speakers. You don't need a high-gain full-stack for country and, in fact, it is detrimental to your sound if you do have one.
If the amp you choose doesn't have reverb, you can supplement that with a reverb pedal or dial in your slap-back echo for a little more fullness. As for settings, as I mentioned previously, you want to put some extra weight in the treble side of the amp and then you can adjust from there depending on your taste.
Now that you have a starting point for how to get that modern country guitar sound, you can play with it to achieve your own sound. Also, I've found that the most important aspect of your tone is your fingers. To sound cliché, it's all in the touch. Learning a few country licks and learning the feel will take you a long way with a little preparation in getting your ideal tone.
If you've discovered other secrets, I'd be glad to hear them and even supplement this article with that information, so please do share. Good luck!