Summarize:

First, check with any friends or relatives too see if they will let you park on their property. If not, check to see if there are any organizations or businesses in your area (or a nearby area) that designates parking lots specifically for people in situations like yours; for example, Walmart allows people to camp overnight in their parking lots. It's not only legal, but the organization might screen the people who use the lot, or even designate a women-only lot. If there are no such lots available, and you live in an urban area, look for streets with no sidewalks, no overlooking windows, and adjacent to woods; the area should be sparse enough to avoid nosy onlookers but populated enough that the car does not stand out. Parking lots of big-box retailers (especially those that are open 24 hours and have restrooms, such as Walmart) are great to clean up in and have security, as long as you spend a couple of dollars there and don't park in one place too often. Parking lots however can be noisy, particularly in the morning as trucks arrive carrying food and goods. Church car parks are often quiet during weekdays. If you check around, you may find a church that is less used than others. This could be a good place to park, and you may be able to ask for assistance at the church. You might even attend the church to gain rapport, but wait a while before telling others about your situation, and tell only those who seem trustworthy and willing to help. Industrial estates and business parks are often noisy by day, but very quiet at night. Small ones close to residential areas are best. They have to be quiet at night. You may encounter security in some places like this, but if you are honest, saying you are just sleeping the night in your car, they usually won't bother you. Their main role is to protect the property. University car parks. This is okay if you are a student, but not so good if you are not associated with the university. You may be required to get a parking permit. Camping grounds are another option, although they usually have time limits and some are almost as expensive as a hotel room. Some offer a shower for a nominal fee. National Forests have some free camping with a limit of 14 days. Yacht harbors are notoriously 'free zones'given the nature of fishermen and boats, so marinas offer a lot of services, like hot showers and transient vehicles. If the season is high, larger boats from out of state show up and stay for months along with their respective crews, all of whom are 'transients' providing excellent cover for you and your vehicle. They don't know or care, and if they find out they still don't care, being a 'little wild' themselves. Hang around on the weekend and meet someone who wants their boat washed and waxedthat'll do it, from there on in you'll have a gate/shower key and legitimacy. If there's no restroom, having a creek nearby helps for rinsing purposes. Know how to safely defecate outdoors and make a poop tube. Five gallon bucket with a lid and lye for odour can also work. A free hospital parking lot is another option. If approached by a guard, you can say that you're waiting to visit a sick relative. However, note that in Australia, due to past murders of nurses, you may attract police attention by parking in a hospital car park. You may be asked to move on by security. If you can establish rapport with the manager of a retail store or restaurant, they may not give you problems about staying overnight, especially if they see your presence as a form of overnight security. Try a hotel parking lot. Hotels and motels along the interstate allow cars to park until 11:00 a.m. the next day (checkout time). As long as your seat reclines fully, no one will notice you. However, you'll need to keep moving on. Once you find a spot, try to arrive late at night, and leave before 7am. This will draw as little attention as possible to yourself. Due to noise, you might find that you will need earplugs to sleep. Earplugs will block a lot of background noise to a level that is bearable. Earplugs are good for blocking out traffic, birds, animals, talking and background music. They will not block out very loud noise or close noise, such as some one tapping on your car.
Find a safe and inconspicuous place to park. Get earplugs.