I get a few compliments on my anatomy. My advice is simple.
Yep, capes and tights, all that stuff. Why? Because in normal, everyday, every one standing around chatting comics, you're doing profile stuff and not much else. Superhero comics encourage you in to doing extreme action poses that really mean you have to study every muscle in a plethora of angles.
"But I don't want to draw super-muscle men"
In some of my characters, I've pumped their muscles up to the max. This isn't meant to be perverse, but rather I will be drawing these guys in almost every conceivable position and gain some good knowledge of how muscles link. I can take this knowledge with me to other projects. Some of the hardest sets of muscles I've learned have been back muscles, forearms and those damn thighs! I feel by my being able to tackle them in so many various positions, I've come further than I would have with just plain life drawing.
And you should try doing a Superhero story
Yup, the problem with just doing random Superhero sketches is that there's a good chance you wont concoct scenarios that will push your limits. I've found when I'm just drawing my characters, it's all profile stuff. Actually having to put your characters in to a far-fetched scenario, getting those amazing 'camera' angles in to your panels, that's where you're going to test your anatomy skills. I can't think of any other genre that showcases extreme anatomy like this.