I prefer to park on-property at the P4 daily parking garage. 1 stop on the AirTrain to terminal C, so there are no busses to worry about, and for on-airport parking the rate is reasonable (IMHO) at $27 per day (don't bother with the valet parking - you'll pay DOUBLE!) Best of all, if there's nasty weather, you don't need to clean off your vehicle (unless you have to park on the roof).