First off, use a mic (or two) always, as JN said.
BUT..if you want your guitar to 'cut through' a mix a bit more, it can sometimes help to mix in a little of the DI signal.
JN has mentioned that it depends a hell of a lot on the type of tone/music you are doing and the density of the mix. I'll back him up on that.
He likes a dead sound and that comes across in his songs, where the guitar sounds up close and personal.
I like a different type of sound. The room you record in plays a HUGE role in recording an acoustic, even when you are after a dead sound.
I'm recording in a local church next week, specifically to do acoustic guitars for an EP. If you are after a rich, reverby sound you could try this. The 'alternative' religions (methodist, baptist etc) are often quite receptive to local arts (I'm not paying anything to use the church for a few hours) and often have slightly more modern churches with a smoother, less 'drowning' reverb than the big old stone ones...
You can construct moveable acoustic panels quite easily to help you deaden a room. I just built 6 and was going to post a little tutorial on here. Depending on construction they can be effective down to about 80-100hz
As is probably obvious, my philosophy is 'try to get it as close as possible at the source'. The better you get the actual recording, the easier mixing will be. You can't polish a...
Mic placement is crucial.
What mic's do you have available? Perhaps we can give tips on using these.