Last year before the tournament I was working on "Twisted Crosswords" by Henry Hook and stopped since I thought I should be doing traditional crosswords instead of these variants. Still, it didn't hurt given the results, and solving unusual puzzles keeps the mind limber.
I picked up the book again, since I've run out of current crossword books. These twisted puzzles can be hard but are brilliantly constructed and a joy to solve. It's good practice to stretch the mind until the "aha!" comes, and to get over the "I can't do this!" panic until I can. Sometimes I got lazy and peeked at answers to unstick myself or verify my progress (my excuse: I was working in pen and didn't want to make a mess).
A sequel "Terribly Twisted Crosswords" is coming in June, and it's on my wishlist.