Parking for the night
You might just need to park for a single night because you are traveling or lost your house keys, or you might be living in your car for a much longer period of time. No matter what the case you can minimize the dangers with a little bit on intelligence.
You are going to want to change parking spots on a nightly basis. Don't keep a pattern to your locations. Driving to your parking location should be the absolute last thing you do before you go to sleep. Don't park at 8 PM and then futz around with a candle and a book for 3 hours, as that will attract attention, and attention is the last thing you are going to want. All you should do is just pull into your spot, climb into the back and go to sleep. If you don't get out of the car and avoid creating any lights then you probably won't attract any attention at all.
If you are parking in a parking lot then you are going to want to park in the vicinity of other vehicles. If at all possible try to select a spot next to a larger vehicle, between two of them is even better. If your car or van is sandwiched between a pair of box trucks (or even two minivans) then it isn't going to stand out, and no one will be able to see into it without effort. Don't make the mistake of parking off all by yourself in a far corner of the lot, anyone doing that sticks out like a sore thumb.
If you are a day sleeper then your choices are nearly unlimited, just try to avoid mall parking lots, as they almost all have bored security guards driving around in golf carts looking for trouble. Night sleepers have to be a little more selective. Your number one choice should be hotel parking lots. Hotel parking lots always have lots of vehicles in them at night and those vehicles are often loaded with possessions. Just try to pick a parking spot out of direct view of the front door, because you don't want the person behind the front counter to see you pull up, but then never get out of the car. If for some reason you can't find a hotel parking lot then park in front of a house on a residential street. Pick a corner house so you can park on the side of the house rather than the front. People tend to get very possessive about the piece of street in front of their house, but not so much about the parking spaces on the side of the house. If you park in front of a house then be prepared to leave early in the morning.
There are a lot of places that you should never park, because they are dangerous or because they are in locations that tend to attract police attention. Never park in the parking lot of any business that isn't open all night. You should also avoid libraries, parks, government offices and all other places that are neither business nor residences. Those places tend to get extra police attention for various reasons (mostly because of the fact that they are favorite "parking" locations for teenagers). Commuter parking lots and highway rest stops are great for day sleeping, but can be dangerous at night, so avoid them after dark. Finally you should avoid the classic "down by the river" spots at all costs, these tend to be favorite party and teenager hangouts and they usually get lots of police attention.