A lot of the options you'll find attach under the joist, which is the easiest and nicest looking solution. I couldn't do that because my joists sit on a cross member that would block the path of the corrugated sheeting, so I went with a between-the-joist membrane solution. If you're interested in that check out Goberco or Trex under deck membranes.

They staple to the top of each joist (after pulling up all the decking) and create a channel down to one end where you can cut a hole and run it into a gutter. I used the Goberco one a couple years ago (much cheaper than Trex) and haven't had an issue.