Look out for old posters that are still up - a sure sign of a site with a long life. Be warned though, that if you start regularly posting up over the big music posters, you may end up getting a visit from some very unsavoury types - that operation is run by some very dodgy gangs who are not adverse to a bit of aggro if they think you're invading their 'patch'. Paste on the wall where the poster is going to go. Put the poster up, press it flat and then paste over it again. This helps to smooth out bubbles, and also makes it harder to rip the poster off.
Some people prefer to flypost in the dead of night, some do it in broad daylight.
This depends a lot on the area you are in, it's up to you. In most areas there are local by-laws against flyposting. Breaking these by-laws is a criminal offence, which means that if the police catch you, you can be arrested and charged and possibly end up in front of magistrate getting a small fine. Make sure you've got some idea of your rights if you are arrested, just in case - this will make you feel a lot more confident dealing with the police. Best made out of lino to last a long time and stay flat easily.