How to get high quality acoustic guitar recordings?

Im a pretty good guitar player but my recordings always come out poorly. I dont exactly have good quality gear, i record with an old cheap acoustic guitar and use the default pre's in my interface. Where should i invest money first, pre's mics or the guitar? If you play guitar and are satisfied with how yours come out, would you mind leaving a sample for me to listen to, and any knowledge about picking gear?

I could write a book... except for the part where I'm too lazy!
 
The guitar track starts with a great guitar part, and a great performance, but you asked about the technical issues, so we'll skip those.
 
Next up is the instrument - not only does it have to sound good, but it has to be set up well, and in tune. Fresh strings may, or may not be required, some of my guitars sound better with brand new strings, some sound better when the strings have been played for a few days. If you are using a fresh set make sure you have them stretched so that they stay in tune.
 
The room you are recording is almost as important as the guitar. Almost, because you can usually work around quirks of the room through careful placement of the guitar and microphone.
 
Microphone selection and placement is important too. If you don't have an array of microphones to select from then it comes down to placement. And both selection and placement depend on... you guessed it, the guitar and the room.
 
I find preamplifier selection to be the least important part of the puzzle, but you can't use jumk, it has to be at least decent. You can use preamplifier selection - if you have more than one - to help shape the recorded sound. Some may argue that the most neutral recording is the best, and I guess that makes sense, but there is also some value in knowing what you want and committing early. (that's fodder for an entirely different thread!)
 
If I were in your shoes I'd rent time at a well equipped studio with an engineer that gets guitar sounds you like... and I'd spend a couple hours auditioning different microphones. I think you'll learn a lot!
 
I don't know that there is a lot of value to my suggestions, but since you asked I'll tell you how I record my guitars:
 
In my current basement studio I can't get away with omni-directional microphones, the room just doesn't sound that good. So I use cardiod microphones, and place them fairly clsoe to the guitar so that I don't pick up a lot of room tone. If the room sounds great I use omni-directional microphones and add some distance.
 
I almost always use a pair of microphones, not so much for stereo, but to provide a couple different perspectives, which I mix to taste later. Usually I'll use a (relatively) neutral large capsule condensor placed above the guitar (ear level), sometimes over my shoulder, sometimes a foot or so in front of me. I also use a small capsule condensor microphone, selected for it's particular sound, and placed anywhere between six and eighteen inches from the guitar, pointed anywhere but at the sound hole itself. Some guitars sound better with that microphone pointed at the neck/body junction, some like a spot closer to the bridge. Every guitar, and every song is different.
 
Microphones I use often for my guitars include:
large capsule - a Neuman TLM-193, Shure KSM-32 or (rarely) AKG C-414
small capsule - an AKG C451, Earthworks SR77, Blue Dragonfly, or Teac PE-120 (modified for phantom power.)
 
Hope this helps.

I think very good mike is essential, I have rode NT1-A and it records perfectly voice and acoustic guitars.
Check video on their website, you can see the placement and other tips.
http://www.rodemic.com/mics/nt1-a

+1 about the room. A treated room is your best start. If you don't have a treated room - you should experiment with different locations throughout the room to see where you get the best sound.

Probably best done in steps;

The first thing is you and the guitar-
Do you like the way you sound? If yes then it's all about capturing it.
If no then you need to find an acoustic guitar that will give you the sound you're after.

I know you said "old, cheap guitar" but that doesn't mean it doesn't sound good...and
that is what you didn't mention.
So to answer your question...If you don't have the sound playing...recording gear isn't going to fix that....get a guitar first.

On mics.... I bought my NT-2A at Sam Ash.  About 5 months later, I found myself in LA at a songwriter's convention. 

One of the sponsors was Gauge Microphones. they had a booth set up where they had their mics side by side with the very expensive studio mics everyone tends to drool over. 

the mics were plugging in and you could A/B back and forth between them and they were plugged into headphones so you could hear the differences in the mics.

I was kinda regretting the NT-2A purchase at that moment. If I had it to do over, I'd have gotten the Gauge but my wife gets kinda ticked when I buy lots of gear..... "How many of those things do you actually need?" Not being a musician, she doesn't understand..... but I could ask her the same question about shoes, or purses, or clothes, or......   

I had a option to compare few ~1000$ mikes with rodes NT and I didn't hear that much of the difference if at all, that's why for amateur use I wouldn't spend that much for ksm44.  

i've used many mics in this category, and the rodents, and the KSM44 just kicked all butt.


it's worth that price range, over just about any mic i've tried in the same range.




for amateur use, there's nothing wrong with a good ole SM57.
and they sell them as cheap as $74 new.


i would record an entire album, using nothing but a sm57, and it would rock.

 - Good guitar
 - Brand new strings (appropriately worn in)
 - Acoustically treated space (or an amazing sounding room)
 - Suitable mic (for acoustic I would ONLY ever go condenser mics [unless for some unusual reason] and you can get a great sound with a cheap mic, but a really nice one does wonders - I use Rode K2 in X-Y stereo configuration and it just sounds amazing. Used to use a Behringer B1 mic which was super cheap, sounded great, but compared to the very $$ tube Rode mics, can't compare. You mostly hear the difference when you start to push the eq a little. The great mics are much more forgiving and let you play with the sound so much more. High end clarity is so much better)
 - Ideal mic placement (I typically aim around 12th fret for the harmonics there which I like. Anywhere can work, but usually just keep WELL AWAY from the sound hole as it's boomy)

Ok I've been busy latley so I'm sorry that I am Johnny come lately here is my advice:

Use a good condenser, the NT1a isn't bad, nor is it the best but it will do the job, point that at the 12th fret, then get an Sm58 and rip the pop shield off and point that at the sound hole.

After recording combine the two in a buss, I think you will be amazed at the sound.  What you are hearing is THE GUITAR, ok I mean your getting the jangly sound of the acoustic and your getting the bass also of the acoustic.

I'm sorry I don't have anything up on any of my music sites to demonstrate this sound and technique but trust me the last acoustic recording I did I used this technique and I was blown away because this is the sound I was alway's hunting for in terms of an recorded acoustic.  It will be my goto technique the next time I record an acoustic as well.

Ben