Hi Scribbler,
You've asked a question to which there is no clear cut answer. I does depend greatly on the subject you wish to select and the background you wish to select it from.
In many cases, you will probably find you use a range of techniques. The ones T Paul has given you should give you a good grounding on which to build.
But as Jeff says, its all about practice. There just is no substitute for it, and as you practice, your judgement of which tool to use for which type of selection will improve.
Good luck, and have fun with your selections.
PS. Try this tutorial from Jay Arraich
http://www.arraich.com/ps6_tips_sselect1.htm