I've found that the key is to keep doing it and eventually you will get to the point where it becomes a habit... try not to berate yourself for missing days and I think it's a bit silly to ride if you are sick, stay home instead of making your co-workers sick... I know this isn't feasible for some people but especially if you have sick days stay home and get better.

If you skip a ride for whatever reason, just remember that tomorrow is a new day that you will likely be around for, so you can ride tomorrow if you can't today... just try to make sure you're riding more than you are not. I've found making my commute as enjoyable as possible makes me more likely to do it, even though I took long ways around busier but direct roads... slow down and smile on sunny days, stuff like that.