I have made an extensive list (with additional information & pointers) of possible overnight places to park. I have tried many of them, but there are also some that I have only heard about people parking at. For your convenience, clicking on several of the stores listed will take you to a page of that store's locations. Here are three sites in particular that are valuable resources for parking overnight:
www.parkfreeovernight.com & www.freecampgrounds.com (includes parking lots!) & http://freecampsites.net/. Also, if you are near California, check out http://desertdutch.org/ (Free Campgrounds in Southern California).
If you are interested in staying somewhere long-term, check out the BLM (Bureau of Land Management) - Their site contains information about Long Term Camping on Public Land!
In populated areas, always remember to be as stealthy and inconspicuous as possible. You don't want to get busted living in your vehicle - Your goal is to blend in and be left alone.
OVERNIGHT PARKING POSSIBILITIES:
Walmart - a very popular place to park overnight! Join yahoo group: walmartrving for lots more specific info or if you have questions. Not all Walmarts allow overnight parking, so here is my advice: Call 1-800-walmart and type in the zipcode of a town you are thinking about staying in. It will give you the phone number to that Walmart - use it to call the store manager and ask if that particular store allows overnight parking.
Many K-mart, Sam's Club, and Camping World stores allow overnight parking, but it is always best to ask the specific store to make sure.
24-hour Businesses - grocery stores, restaurants, health clubs, laundromats, manufacturing companies (that have a night shift), department stores, etc. My favorite 24-hr. spot to park has been Meijers. If the store lot you would like to park in isn't open 24 hours, I have heard about parking there with a "For Sale" sign (with a random local phone number) in the front window. Pretty unique idea! Just make sure that nobody can see you inside thru the windows, in case they come to check out the sign.
Casinos - a great place to park overnight! You can always rely on parking in a Casino lot, I've done it several times. They are 24-hours and you can blend right in if you are living in a car or a van. If you are in an RV, go to CasinoCamper.com to find "RV Friendly" Casinos. You can also read tips there about Casino parking "etiquette."
Highway Rest Areas & State Welcome Centers - fairly good spots (plus you can wash your hair in the sink...). You may find the Interstate Rest Area Guide to be a handy resource, since it lists rest areas & welcome centers, organized by state.
Airport Terminal Parking Areas - I haven't done this, but I know that it is a possibility. Be aware that there might be a fee, depending on the size of the airport.
Some Shopping Malls/Plazas - look for signs regarding parking regulations
Hospitals - (good every once in a while, but I don't recommend parking at them very often)
Parking Garage - parking here will cost you something, but it might still be handy in a busy metro area that has limited parking. It would be a good option if you want added security. I've done this overnight in South Beach Miami and Chicago.
Apartment Complexes - look to see if people have parking permits or passes hanging in their vehicles. If not, these can be an excellent place to park overnight. It is best to park off to the side of the lot, but near other vehicles. Try not to be near the main entrance or windows. Don't stand out like a sore thumb, and park in a direction so that people aren't as likely to see in your front windshield. The IDEAL parking place would be a combination parking area, near both a 24-hr. store AND an apartment complex... It would look like you could either be living or visiting someone in an apartment, or working in the store.
Hotels (medium sized) - larger hotels may do frequent license plate security checks, and tiny motels could easily notice that you are there. Pick a medium sized one and feel it out. Hotels are not always recommended, but I have parked in several hotel lots with no problems. I like them quite a bit actually, but I only stay in them for one night and then move somewhere else.
City Streets - some cities allow overnight parking on their streets. Look for signs. People do tend to wonder about unknown vehicles parked outside their homes, so I'd recommend usually only doing this if there is an apartment complex nearby or if you know someone that lives on that street. If neither of those are the case, and you'd still like to park on a street in a residential neighborhood, try to get there pretty late and leave early in the morning. (just go move your vehicle into a grocery store lot and go back to sleep) Also try to park between two houses so it would make it more unclear as to whom you may be "visiting."
Parks/Campgrounds/RV parks - parks can include county parks, city parks, state parks, or national parks. Any of these options will probably require a fee, so they are fine if you don't mind paying for a few luxuries. Every once in a while they are nice. If you are car/van/RV/or even tent camping, definitely check out the U.S. National Forest Campground Directory. It's very informational!
County Roads/Scenic Overlooks/Natural Areas - free, but a little more risky. Not always the best, but definite possibilities. If you are a little ways off a main road they can be pretty nice.
"Dispersed Camping" on National Forest Land - this is free, but you will pretty much be in the middle of nowhere. If you are interested in doing this, look up a national forest in your area. Ask a local ranger where the dispersed camping area is located. Here are 3 sites to help you find a National Forest - U.S. National Forests and Forest Service Guide, US Forest Service, and Find a Forest by State
Federal Wildlife Refuges - (not "Wilderness Areas," which is where vehicles aren't allowed). I haven't parked at one, but I've heard of people doing so.
College/University Campus - there may be a visitor parking area near the student dormitory lots or somewhere else on campus. The best spot would be in a combination lot between the campus library and the student dorms. Campus libraries typically stay open really late and open pretty early in the morning. It could then look like you were either studying/working in the library or working/living in the dorms.
Friend or Relative's Driveway/Yard/Property - by far the best option for overnight parking! Not very possible, though, when you are traveling.
some "Cracker Barrels" - I've never parked at one, but I've heard that some Cracker Barrels do allow overnight parking. If they do, I guess it is usually behind the store, where there are long, designated bus/RV parking spots. Certain "Cracker Barrel" highway billboards have a tiny little bus silhouette in the lower, right-hand corner, which means they've got the designated parking spots. Check them out! They are pretty cool. My recommendation? Always ask a Cracker Barrel first.
Your Job - will they let you park there overnight? It never hurts to ask (unless it will get you fired)
Private Property - if you have permission to do so, it is definitely very safe to park on private property. I read about a guy that lived in his vehicle, traveled a lot, and had excellent references. He would ask real estate agents about opportunies for house-sitting/farm-watching/commerical-sitting/"caretaking" (outside, just "keeping an eye" on people's property). Every once in a while I guess he found a place to act as a temporary property "security watch." Plus he got paid! I wonder how likely of an opportunity this would be.
Large Farms/Ranches - I probably wouldn't do this, but it is similar to the last one I wrote about. I have read about people who ask large farms or ranches in the west if they could just park there for a night...
Truck-Stops - I HIGHLY recommend truck stops. They aren't just for truckers anymore, as RVer's give them lots of business. This is why many truck stops have changed their names to "Travel Centers" or "Travel Plazas." Truck stops can provide all sorts of services... laundry, showers, food, gas, ATMs, Wi-Fi internet, etc. When you are on the road, here are the top two sites I have found for locating truck stops (with phone numbers listed!):
MANY truck stops have a designated auto/RV parking area - and if they do, you don't have to ask to spend the night. Don't park among the truckers or in their designated area! Park in the auto/RV area only. Here are the Truck Stops that are most well-known for welcoming RVer's and smaller vehicles...I have personally read MANY positive reviews about various vehicles parking overnight at both Flying Js and T/As. I personally LOVE parking at Flying J's, Pilot Travel Centers, and T/As. They are great.
Always welcoming and friendly to any vehicle. #1 Recommended Truck Stop.
They "have RV parking spots at locations from coast to coast"
These four also often have auto and RV parking.
Petro's site specifically mentions separate auto parking.