What's the best way to progagate house plants from cuttings?

I get mixed results. Sometimes the plant cuttings florish, sometimes not. Generally, I put the cutting in a cup of water and wait until it develops roots. Sometimes, though, they rot. What am I doing wrong? What are some other 'greenthumb' tips? I'd like to be better with plants.

What's the best way to progagate house plants from cuttings?
There is a white power called rootone or root tone anyway you cut a circle around the bark portion then another about an inch away...cut a line between them and peel off the outter layer. spread rootone over the entire area then dampen spagram moss and wrap a ball of it around the branch over the rootone. Wrap and secure the moss with clear wrap making it as air tight as possible. Roots will grow while the branch is still living off the plant. When you see plenty of roots emerging through the moss clip it off and plant. In the case of say fruit tree cuttings the clipping is as mature as the mother plant and will bare fruit just as it would on the mother plant.
Reply:use rooting hormone powder,, and quality soil
Reply:Egg shells crushed in water.
Reply:Try damp sand or perlite instead of water -- many things need the oxygen they're not getting in water. Also the success rate of transplanting rooted cuttings from sand or perlite is higher than cuttings rooted in water for many species.