Rieder says that powder isn't the worst thing, but it's not particularly effective for extreme sweaters. It should be used sparingly.
An example of good places to use powder is, "in between folds of skin, and between the underside of breast and chests. Be careful, because when these areas get super sweaty, you can develop yeast and fungus and become a petri dish for bacteria," Rieder says.
There's a great anti-fungal powder called Zeasorb that's affordable ($4.50 on Amazon) and works wonders.
But also remember to talk to a doctor to see what's right for you.