LISTEN TO YOUR BODY.
I have a HORRIBLE cold right now (why else would I be on this site?!) and I realized that this is the one time I feel like my body has control over me, not the other way around. And it's probably because I wasn't taking good enough care of it to keep it going like I was!
The two most important things you can do are (1) SLEEP, and (2) DRINK...a LOT. I mean like there will be no water tomorrow. If you're like me, you probably get a fourth of what you're supposed to each day. Which means now is the time to make up, because your body is punishing you. :-)
Aside from that, take vitamins (that, again, you should be taking every day). Boost up your immune system with things like Vitamin C (OJ!), Zinc, and Selenium. Go with the Nature's Way echinacea if you would like, but make sure to give your doctor's recommendation dibs.
Also, stay away from work, school, or other places where you might spread your ickyness. Not for too long, but remember there are other people out there (people with kids!) who will be very unhappy with you if you cramp their style by getting them sick.
Keep the foods you eat pretty bland (is this the rationale behind chicken soup and crackers?) so that you don't put your digestive system through the ringer. The key is nice 'n easy.
I am not sure of this, but I was always told to stay away from dairy products (milk, cheese, etc) as they contribute to the production of phlegm. I'm in the draining process now and don't need any more mucus, thank you.
Take the time (when you're not sleeping) to catch up with movies, books, or online shopping (my personal favorite!) to keep yourself indoors. Try to keep your physical activity to a minimum, but depending on how severe your cold is, this might not be a problem. My body protests every time I try to climb a flight of stairs (and I live on the 3rd floor!) so taking it easy is not difficult for me.
This is just me, but I like to stay away from over-the-counter medication (unless I need it to sleep at night - NyQuil is my legal drug) because I think I can better gauge if I'm getting better without it.