As with so many mixing issues, it's impossible to set hard and fast rules as so much depends on the desired end result. It's generally true that too much processing can easily destroy the natural sound of an acoustic instrument, although that's fine if it's what you want to do of course! Some instruments are more easily 'fixed in the mix' than others, but with acoustic guitar it's definitely better to take your time, play with your mics and their placement, and get the sound as good as possible in the first place. If treatment is required though, read on.
Light compression can help the guitar sit nicely in the mix - use a fairly slow attack so as to avoid squashing transients, and try a ratio of around 3:1 to start with. Additionally, EQ can be used to cut down any resonant frequencies in the guitar or room. These will often be found in the 100-250Hz range - use a parametric EQ to find and then attenuate the problem frequency. Set the mid band to a high boost and narrow bandwidth (Q), and then sweep across the spectrum until you find a sound that really roars - this is the resonant frequency. Cutting this frequency slightly should result in a more evenly balanced sound. Be careful however - by its nature, resonance occurs in narrow bands, and using a wide band of EQ for this purpose can often result in a worse sound than you started with.
The guitar overlaps with the frequency range of the bass guitar - in fact the latter only has the bottom octave to itself, and the two can often seem to be fighting for space within a mix. A gentle bass roll-off starting at around 160Hz usually solves this problem. Again, subtlety is the key - too much EQ can easily make an acoustic guitar sound horrible. If the guitar is heard exposed for any part of the track, you may want to de-activate the roll-off for that section using automation.