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.
Some charge for parking after 2 hours - there are no attendants but they check by CCTV and send you a penalty notice of £100 if you don't pay, Watch out for this. Cheaper to hop into a Travelodge.
If you do find one with low or no parking fees, then you'll be fine. Try to park near others rather than in an isolated spot. Lock the doors and stay safe.