If you hang them upside down in a dark, dry place, they should do okay ... a closet is often good. For best results, there's a sand-like substance that people use, but the flowers should be at peak for that.

Another option is pressing some of the petals - just put them in a heavy book and leave them for a few weeks - they'll end up dry, flat but usually keep their color just fine, and you can arrange them nicely on a piece of paper for a keepsake.