I recently bought a resistance band. I won't know for sure how well it's working until I get out and ride this summer, but so far I think it's helping.
I'm a soccer player, so my limiting factor when wakeboarding has always been my arms. Since I don't get to wakeboard all that often, I hate getting out there and spending half of my second set and all of my third set just struggling to hang on to the rope. So I've been thinking a lot about easy (and economical) ways to build up the wakeboarding muscles in my arms. Wakeboarding more regularly would be the ideal option, but I don't own a boat and have to rely on others. Others on this forum have suggested rowing as good cross-training. So if you own a row boat or a rowing machine, that might be a great option for you.
I decided to try a resistance band due to the low cost. I just attach it to a door with the door attachment, and then I pull on it (either doing reps or just holding it still) while standing in various wakeboard positions (regular, switch, side slide, ts, etc.). Other things I've done is attach it really low on the door and do "rowing" that way. Also you can pull it back with some tension and practice some basic spins with or without your board on.
No idea how much it's going to help, but so far I'm optimistic that it's going to be worthwhile.
If you're more concerned about your legs, go play soccer.