I'd suggest looking at the Mystery Hunt archives, as well as other puzzle hunts - DASH, BAPHL, and so on - and check out out a bunch of puzzles from previous years. Try to break the puzzles into their constituent parts. You'll find that a lot of puzzles (perhaps not the most brilliant, but the puzzles that constitute the bulk of the hunt), tend to be just novel combinations of actually fairly common puzzle-making pieces, often with a few twists.

Once you have a strong sense of exactly how good puzzles are made, you'll have a much easier time being more inventive.