Over on the Quality Chess blog, Jacob Aagaard has written a post titled "Ten ways to improve in chess". There's nothing earth-shatteringly new there, but that doesn't matter. What does matter is that the advice is very good, and any non-professional who follows it is bound to improve. (Some professionals might improve from it as well, though I'd imagine most of them already follow most or all of those tips.) I like his 11th tip a lot as well, and I'd emphasize the second option there as most of us choose the first option automatically.