Well, here is my opinion :
1) Flushing
Previously, I used to flush my engine every alternate oil change. But after that, i read on a site, that flushing was the "old" way, as engine oils nowadays can work properly w/o flushing. Ability of the engine oil is determined by how much "soot" it can hold. When flushing, it might spoil piston rings, as it "strips" the oil off.
2) De-carbon'ising
Haven't tried it before, as I don't know how this product actually works. Saw pictures of cars having this done. There was a lot of white smoke coming out.