Cocca Butter does help, and also helps prevent expansion and formation of new marks. Some kinds are better than others. When my wife was pregnant, we experimented with this. There is a kind of Cocoa Butter that The Body Shop sells in a deoderant like stick form, and it is hands down the best type of Cocoa Butter we found. Plus, since it is in the stick, it is easy to apply.
Also, i havent used retin-a, but am friends with my dermo who used it on himself for 6 months - he said that he thought it was about worthless. He is on to trying laser treatments now. He thinks this is better, but overall he says that basically with stretch marks, one you get them, you have them. He also says that different parts of the body have responded differently to treatment. The parts he was trying on was the bicep-armpit-outer chest area.