Electronic tuners: It wasn't long ago that guitarists had to totally depend on their own ears to keep their guitars in tune - what a radical thought for younger players, eh? Whether it involved a tuning fork, pitch pipe, a fixed pitch instrument to tune to like a harmonica or a piano, of just tuning the strings to each other, it was all about how good you were at differentiating between pitches very close together. An inexact science at best!
Today we have a huge array of inexpensive digital tuners that are inexpensive, accurate and easy to use. For dinosaurs like me, the idea that you don't even have to actually hear the guitar to get it in tune is absolutely revolutionary! My point is: today there is NO REASON to play with a guitar that is out of tune. If you don't already own one (or would like to try a design that is different than the one you already own), this is a great item to ask Santa for!
Acoustic guitar pick-ups/Acoustic amps: I remember my first "acoustic guitar pick-up." It was nothing more than a humbucker designed for use on electric guitar, with a spring mechanism to hold it in the sound hole, and a wire that dangled off the side. It was awkward to use, scratched the edge of the sound hole on an expensive guitar, had the annoying habit of falling into the guitar, and most importantly, sounded like someone had stuffed my guitar full of socks. Yuck.
Then came the first generation of piezo-type pick-ups the worked by amplifying the vibration of the instrument. They sounded abrasive and "squawky" and were prone to horrible, self generated feedback just about the time you went to the bathroom between sets and the guitar was sitting in its stand on stage.
But the companies that made pick-ups and many newcomers realized that what guitarists wanted was quite simple: an acoustic pick-up that just made their guitars louder, with a natural sound and minimal feedback issues. The result was the new generation of pick-ups, some pre-installed in guitars from manufacturers and some available as add-ons. Most of them sound terrific and their sound is easily tweaked to get just the quality you're looking for. Some featuring "modeling" to make your Martin D-28 sound like a, say, Gibson J-200. Amazing. And priced reasonably too!
As far as amps go, designers are coming up with wonderful sounding units. We no longer have to depend on amps designed for electric guitars (high impedance; prone to feedback) or having to plug into a big PA system. There is no question that a microphone, especially a good quality one, will always give the most natural sound if you need more volume but mics are very restrictive and must placed absolutely right for the best possible sound. And then there's that feedback issue...
There are dozens of amps on the market that are specifically designed for acoustic guitar. I use a Carvin AG-100D (with a tube pre-amp) and I love the sound it gives me. The Roland AC-33, -60, and -90 are good choices too, as are some of the amps from Fishman. If I could afford it I would buy the Bose L1, which is an absolutely amazing amp that sounds incredible and features zero feedback issues, even if the sound column is placed directly behind the guitarist.
Bone nuts, saddles and bridge pins: My last fave items are decidedly low tech compared to the previous two. All acoustic guitarists want their prized guitars to sound as good as possible and the best way I know to make this happen is by replacing the standard plastic saddles, pins and nuts with bone ones. Some very expensive guitars still come through with plastic, which amazes me. If you own one of these - check the manufacturer's spec sheet to be sure - buy and install a new bone saddle and bridge pins to start, and if you don't mind spending just a few dollars, take the guitar to an established luthier and have him or her replace the nut with bone. Bone is a much denser material than plastic, therefor it transmits the vibration of the strings better, which lets the instrument vibrate more - and produce more resonance and volume.
You may have to sand down the under side of the new bone saddle a bit, which is easy to do but do a tiny bit at a time and re-check the string height often. I always do this with old strings that I'm going to be changing anyway. Bridge saddles should NEVER be glued in, by the way.
You might also want to try a man-made material called Tusq, which many people feel is just as good as bone.
You'll find a great selection of all these items at Stewart-McDonald and through Bob Colosi guitar supplies.
Peace & good music,