I'm not going to discuss this in depth as I'm not actually a pro at this...but a while ago I asked about this concept via Twitter and I received a huge response - people who'd saved for a whole holiday through these sites. So I'm going to give it a go.
It's hard, as you have to almost re-train your brain to remember to go through these sites but once you're used to it, it's second nature. And it does mean you have to sign up to a whole new set of sites...but I have never-ending streams of junk anyway!
Top Cashback sites (free to join):
1) Quidco - retailers include Next, Asos, Debenhams, M+S, Tesco (and you can get it as an App, too)
2) Top Cashback - retailers include Expedia, Argos, O2
3) R Points - retailers include Boots, Boden and M+S