I like to hunt field edges or clearings when its raining. As crazy as that might sound, turkeys head to these areas when its raining. Im not talking about monsoons of course, but a light or steady rain. There are lots of theories on why turkeys head to the field when it rains. I dont get caught up in all that, i just know they head there, and its a great place to hunt in those conditions.
As far as how to hunt them in the rain, set up in the areas where they enter or exit the field. Use decoys and calling just like you would in any other situation.
Another tactic is to set up in those same areas without decoys or calling and let the birds come out naturally. An ambush style hunt. When calling and decoys dont get it done, this can be one of the most effective techniques...Just be patient...They will come.
Best of luck.