'Tis the season when the media puts out a bunch of well-intentioned articles and reports with stereotypical suggestions for people to avoid getting colds. Ignore them, and follow mine instead.
1. ALWAYS avoid contact between your hands and your eyes. Around 80% of colds enter through the eyes and from there proceed to infect your sinuses (giving you the characteristic sniffles). It's a general rule of thumb to avoid touching your hands to your face (especially your nose), but avoid your eyes like the plague. Use your sleeve instead.
2. If you wash your hands, re-innoculate them with bacteria from your upper arms, neck, or lower legs immediately. You're not a surgeon, and your hands don't need to be "sterile." No matter how hard you scrub, you'll always have things living on your hands, so your goal should be to keep benign bacteria happily reproducing there in order to physically compete with and starve out the tough, pathogenic stuff that you will regularly encounter in the wild. Never, ever, assume that "germs" are inherenty bad.
3. Get plenty of sleep. Okay, so this one's probably not new to anyone, but nobody seems to actually follow it. :P It's right up there with "don't stress the hell out over trivial things," but then again, it's America. Long story short, your immune system very quickly starts to go nuts if you don't get enough sleep. It only takes a couple of days of poor sleep for Bad Things to gain a foothold. If you find yourself getting abnormally tired, a combination of going home, blasting the heat, and sleeping can be the difference between two weeks of semi-sick hell and two weeks of normality.
Anyway, that's about it. Sure, you'll hear a bunch of stuff about getting exercise and avoiding sick people, but many of the common infections are most readily transmitted when the person infected with them is asymptomatic (i.e., not apparently "sick"), while the exercise part is only really important when it comes to avoiding infections if your diet is either too high or too low in simple sugars (and, well, let's face it, if you live in this country, your diet is probably high in simple sugars). Either way, I consider exercise and avoiding dudes hacking up a lung to be common sense, so they don't get their own numbers. :P
Cheers.