i have a martin acoustic handed down to me by my grandmother. its starting to look a little dirty/less shiny than it did when i got it. what do i do to clean it and get it looking new again?

Well, aside from polishing the body,fretboard, and strings with a clean rag every so often to wipe away built up hand oil, sweat, etc, what I do is put just a tiny drop of 3-in-1 oil (available at any hardware store) in each groove on the nut to help prevent the strings from cutting into it. That aside, just make sure it's stored in a case so humidity doesn't affect it too much. If you're really concerned, you may look into buying a humidifier or some humidipacks available at any large guitar store (guitar centre comes to mind). If you really value the guitar and you don't have one, be sure to invest in a hard case for it. AFAIK that's about the meat of it.

Hope that helps
thanks! as far as polishing, is the polish compound availabe at a guitar store? what do i rub it on with, just a rag?

thanks again.

ps. i do keep it in a hard case when im not using it
Best is just a clean soft cloth, damp if it's really dirty. DAMP, not wet, I use old t-shirts when they start to wear holes in them. Cut out a cloth and keep it in the guitar case, or hanging on the guitar stand. Once it's clean a dry cloth every few days should keep it in good shape.

I don't like guitar polishes for acoustics, I don't want anything added to the guitar or finish that might soften the finish, interfere with resonance, fill in tiny cracks and become visible etc. A camera brush is handy too for getting dust off places like tuning keys, corners, bridge just under the edge of the strings...
Hmmm...I wonder what this button does...
Actually the best is using a microfiber cloth. If you just a normal rag, you chance scratching your guitar b/c the fibers are not fine enough. Normal rag = shimmy cloth, old t-shirts, regular towels etc.

You can find microfiber cloths in the automotive section at Walmart. Same cloth they use to polish cars (doesn't scratch the car's finish either).
I don't like those car microfiber cloths like you can get from Wally World...they give off too much lint. I got a couple of them a while back and they suck. I use a lambskin shammy. Get it soaking wet, then wring it out like crazy. I mean wring it out until it's nearly dry again. That will remove most all of the smudges and so on from the wood. I even use it to wipe down my strings from time to time. As far as polishes, I use Gerlitz guitar products. Good stuff, inexpensive, can be ordered directly from them, and formulated specifically for guitar use. The two I use are Guitar Honey for the fretboard, and their N0. 1 guitar wax, pure carnauba wax, for the rest of the guitar. It protects the shine very nicely, and only needs to be done once every few months.
As far as humidity goes, try to keep the guitar at approx. 45% at all times when not playing it. There's a recall on the Planet Waves Humidipack, so until that mess is cleared up, it's best not to use that. They will notify the public when a suitable replacement is available.