As I consider myself some kind of an expert on this topic I decided to write some tips and tricks down.
To me the most important part of stealth camping (or wild camping or free camping) is how you feel. You need confidence to sleep well out there in the forest or wherever you are. To me wild camping also brings a lot of joy, brings me closer to nature and gives me a great sense of freedom.
Where: In my opinion you can stealth camp basically everywhere. I have camped in the main square of villages, in parks in cities, under big highways, in empty buildings of all kind, in the forest, in sheds, in peoples back yards, on farmland, on the beach, in churches and monasteries, in peoples garages, in closed camp sites, on jetties, in bus stops, behind supermarkets, in industrial areas, in schools, under a parked truck, etc. I do prefer the quite places in nature and that is where I spend most of the nights. Have a look at your map for a green area, stock up on food and water, cycle to the green area, walk 100m of the road and set up camp. As you go you will develop an eye for good camp spots.
When: It is good to time well the time of arrival and departure from your camp spot. In nature, if you have a place well hidden or in a quiet area it doesn't matter. But if you are in sight of a road or on private land it is probably better to set up your tent around sunset and pack not more than an hour after sunrise. You can still spend more time in the place, but because free camping is illegal in most countries it is good not to have your tent up. In the busier areas I usually cook and have dinner first before pitching my tent. If you want to ask people to camp in there back yard or garage it is better not to wait until sunset. Most people don't like you to ring their bell in the dark. Also it still leaves them time to invite you for dinner:) When I camp behind supermarkets I usually have a look at the opening hours and leave well before that. Industrial areas are best in weekends.
Tent To me the single most important feature of a tent is that it should be free standing. I estimate that I camped at least 100 times in an empty building or under a roof, just using my inner tent. Because of the concrete floor it was not possible to use pegs. Another thing to think about is the color of the fly. Mine is dark green, fantastic in almost all nature, almost invisible in the dark. Once it took me about an hour to find my own tent from the place where i had dinner, about 100m away because of bear risk. Of course the tent has to be wind proof, especially in high mountains, big deserts and on the beach. You will be out there alone and you have to survive storms by yourself, without help available.
Alternatives: In extremely populated areas it can be hard to find a place. If you don't like the supermarket and industrial areas idea. Of course we can use couchsurfing or warmshowers. Also you can ring bells, talk to people in the street, etc, but this turns out to be difficult in cities. In South America I have heard of people staying with fire brigates a lot, myself i stayed with police a fair bit. Try if a church is unlocked or look behind it for a place. Look around for alternative looking people and ask for a squat. I think with all this kind of ideas single women will be more successful. In many train stations and parks quite some people are sleeping every night. Not just bums, also other travelers.
The first few times I was stealth camping I was a bit nervous. Soon I discovered that there is almost nobody out there at night and if there is somebody walking his dog or a shepherd or something, they are more scared of me than I am of them. In about 1000 wild camping experiences I was never chased away or hassled in any sense. Which is pretty amazing. Some people are afraid of wild animals. Close your tent for spiders, scorpions and snakes. In bear country take the normal procautions. The other large animals you are not very likely to see. Dogs will be your most frequent guests. Once i heard wolves and once a wild boar.
Start the first time in a forest or in the mountains where it is easy to hide, soon you will gain confidence and extent your terrain to other places.
I am very interested to hear from other peoples experiences or opinions.