Check out the signs - some require payment for more than two hours parking.
If you are just having a nap, pack a sleep mask, because ideally you want to park in a lit area, and that can stop you dropping off. Otherwise, cover your eyes with a hankie.
Open your passenger window a tiny crack to let some ventilation in.
Although it`s obvious, you`d be amazed how many people forget - take your keys out of the ignition and put them out of sight, but on your person, and lock all your doors.