We found this cute infographic (for a larger version, click here) courtesy of the collaborative community over at GOOD, but we checked with local Portland bike shop, Gracie's Wrench, on some of the specifics just to be sure we were relaying good advice.
The founder, owner and main mechanic at Gracie's Wrench, Tori, pointed out that while these tips are pretty good, she "would get rid of the derailleur advice and instead include a tip on how to find a good bike shop ... Most of the things [included] are things the average novice rider might be able to do, but the derailleurs are way beyond that."
Tori also pointed out that new-ish riders might not know what sort of things are actually right when it comes to tightening axles, truing wheels, saddle height, etc., so unless you really know what you're doing, scope out a local bike shop to get some help. It's a good idea to get your bike tuned up every 6 months to a year, depending on how often you're riding, anyway.
And BicyclingHub.com's own little tip: wear a helmet while you ride.