I've been told by my professor to "take the C trumpet challenge." This means that whatever I'm doing, be it solo work, etudes, exercises, quintet, band, or even just warming up, he expects it to be on the C and transposed accordingly. This can sure make you feel stupid sometimes, but it's worth it! My chops are getting so much better on both horns (I've only had a C since last summer) and my transposition is improving, too!
I think I'll wait a year or so, though, before I take the "Eb trumpet challenge!"
Jeff