I've been looking into this recently. I've registered with a few survey sites (Pure Profile, Ciao etc) but haven't looked into how often they pay out. Have also registered with some of the cashback sites (greasypalm, myshoppingrewards, pigsback, quidco etc) and built up a small amount of cashback by clicking on certain websites, taking out offers with certain sites, and if you are buying presents online then if you use certain shops you get cashback there as well.
Wouldn't really recommend anything like envelope stuffing/labelling as these are often scams and don't end up paying hardly anything.
You could also try mystery shopping, not got any contact details but that sometimes pays OK, especially if you might be going to the places anyway.