A guitar that is used regularly will sooner or later show signs of wear and tear. How do you best care for your guitar and make sure it stays as good as it is for as long as possible?
Every once in a while, I polish the main body with a soft cloth to remove dust and smudges.
When changing strings I give the fretboad, headstock and pickup areas a good cleaning.
A can of compressed air is a good idea for removing dust and other gatherings from tight crevices like the gaps between the pickups and body. Make sure you hold it at least 6 - 8 inches away though.
I polish the fretboard with a brand of polish I get from my local music shop called 'FretFast'. You might be able to find something similar.
Occasionally I'll tighten the screws around the guitar if they're coming loose. I tend to find that the jack-socket on my guitars come slightly loose after a year or two, so I tighten that too.
As for scratches and/or chips, I haven't done anything about them on my guitar. I have one particular chip in the paint on the bottom of the guitar near the jack socket. I'be had suggestions to repaint it with nail polish or something similar, but I'm still hesitant.
I have some basic maintenance regimes I follow based on as-I-play, weekly, and monthly schedules--along with spot fixing any serious problems I come across as I play my arsenal. I feel like the more instruments you own, the less organized you can be on this and therefore the more problems you end up fixing as you go. Wood moves, screws come loose, and strings age--so you ultimately need to at least check the basics on a semi-frequent schedule. Here's my basic run down for electric guitars:
Once or twice a month
Check all of the pickguard screws, pickup rings, jack plates, potentiometer nuts, and tuning machine screws and lugs to make sure they are tight. Takes less than 30 seconds on my Les Paul--and that's the most complicated guitar I own hardware-wise.
If I'm in need of a string change (and I usually am) and the fretboard is nasty, then remove all the strings and hit the fretboard with some naptha (common lighter fluid) to clean the gunk off followed up with some Gerlitz Guitar Honey--which is basically mineral oil. See this question for some more strategies on how to clean your fretboard.
Check the potentiometer travel for all volume and tone pots making sure to listen for any scratching (obviously you'd need to plug the guitar in for this part). Also, make sure all the volume knobs are secured tightly and won't come off through normal use.
Quickly check pickup height with a small precise ruler (kept in my case) to make sure that the pickup screws have not vibrated loose from my constant abuse throughout the month.
Check to see if I need to replace strings, and if I do change them out one string at a time. Some people swap them all at once all the time, but I have found that the setup on guitars equipped with Tune-O-Matic bridges are easily disturbed, so I don't like to do this unless I need to clean the fretboard.
Wipe down the entire body and neck of the guitar, paying special attention to nooks and crannies like underneath the strings and the headstock--no polish though. I have had bad experiences with polish in the past, so I simply use a little distilled water and some naptha for the hardware.
After I Play
Wipe down the front and back of the guitar with a soft cloth when I'm done. I also attempt to prolong the life of my strings by wiping those down as well with a separate cloth. My fingers are quite oily, so I can decimate a pack of strings in less than a week if I play frequently.
Humidity and Storage
Relative humidity is important for any wooden instrument because wood absorbs water, so I make sure I have a humidifier handy in the winter months to make sure that my playing room doesn't get too dry. Dryer atmospheres can potentially cause finish damage on nitro-cellulose and polyurethane finished guitars as the wood shrinks. Anywhere from 35% to 50% are good numbers for acoustic and electric guitars. Don't make the mistake of thinking that humidity only affects acoustics though; electric guitars are made of wood too.
Since I have a couple of nice instruments, I usually keep the serious guitars that I'm not playing in their hard case--but cheapies like my favorite Tex-Mex Telecaster are often found propped up against my amplifier for quick access :D.
I use a sound hole humidifier because of the climate I live in (Lifeguard, in my case), but not everyone will need that.
I make sure to wipe down the strings, fretboard and body after each practice/playing session. Strings last much longer and sweaty grime does not accumulate on the body and neck. I have never polished any of my guitars (I have an acoustic and a classical) but the bodies are still as shiny as when I first got them! Never skip the wipe-downs!
My bass, on the other hand, gets more attention, as I play it so much more. Besides the wipe-down, I adjust the intonation from time to time and also make sure the saddle height is not too low. I have a set of precision tools that make this quick work. I also have to tighten the strap nut screws often, as those get loose after a lot of playing.
keep it in its [hard] case when you're not playing it.
I'm a terrible person - I leave my guitar on its stand next to a radiator when I'm not playing it. This is a bad idea since the constant changes in temperature will make your guitar go out of tune a lot and may damage the glue used in the guitar (my excuse is that my room is small).
Putting your guitar away in a hard case rather than on a stand will help to prevent bumps and knocks (horror story: my friend left her guitar on a stand; it got knocked over and a tuning peg came off) and also prevent it from getting too dirty.