Well, chain wear is almost entirely caused by dirt getting on the chain and working on it like sand paper. So, the best way to get a chain to last longer is to clean it frequently (of course, you have to lube it after cleaning it).

I keep my chains reasonably clean, giving them a thorough cleaning and lube every 300 miles or so. That gives me a good 4,000 miles out of a chain. Now, I read one guy post who cleans his chain after every ride (the logical extreme) and he got something like 15,000 miles of wear out of a chain.

You can determine whether a chain has stretched by measuring it. 12 links should measure 12". Different people have different ideas of how much "stretch" can be tolerated but I change mine when it gets much over 1/16".