To keep a bike in good shape especially if you are a commuter or hauling heavy cargo *cough*your 27 pound kid you should bring it in every couple of months. It isn't a bad idea to plan to take it at the start of every season. This is great if you are winter rider too so the bike can be set up for snow or in the spring for rain. For occasional riders once a year should be fine, if you are a warm weather rider before and at the end of your season would be best.
In the mean time follow these two rules keep your bicycle sparkly and running well. As recommended by our LBS aka check with your own shop as I am no mechanic!
1. Inflate tires to the psi stamped on their side at least once a week.
2. Lubricate your chain at least once a month. (not required if you have a closed chain)
PS: Give the bike a visual inspection. Just knowing what your bike looks like you are more likely to know if something is off.
My favorite guys making sure our bikes are ready to ride.