When I'm in major cities or when I don't have friends or family around in the area that I'm visiting, this is more or less my approach:
Use common sense.
Look at street / parking signage very carefully and repeatedly.
Get ready for bed before you approach the night's parking / sleep spot.
Don't get in and out of your vehicle or open and close your doors.
Don't idle your engine, watch loud movies on bright screens, turn lights on and off, blab on the phone, or really do anything that might indicate that someone is inside your vehicle.
Do your best to avoid parking in the same spot for more than one night.
Move around. Switch it up. Don't be memorable.
Never leave a mess around your vehicle, EVER. Even if you didn't make the mess, clean up around your vehicle, ALWAYS.
If the area around your vehicle is such a dire mess or if your vehicle is generally an eyesore, clean up the area and consider investing the time and money to beautify or artistically unique-ify your vehicle, especially if you wish to peacefully park in beautiful areas. These areas often have very fancy homes and very fancy people around who may be displeased by the presence of your creepy bucket of rust on wheels with fast food bags, and clothes and trash all up in the dash.
If you get a knock and you're drilled about what you're doing in your vehicle, explain that you're just traveling through the area and kindly ask for a suggestion on where it is safe and allowed for you to park for the day.
If law enforcement become aggressive, kindly explain that you will be on your way, far away, immediately.
Drive to a Walmart or large retail store where many other vehicles are parked overnight.
Lastly, don't be afraid to ask a business owner or employee if it is ok to park your vehicle in their lot overnight. Asking property owners solves so many problems all at once. You now have permission and the "locals" are aware of who you are and who your vehicle belongs to. Win-win.
So tell me... Where do YOU park?
If you often stealth camp in major cities, what are some of your strategies?