Got hold of Arban's Complete Method for Trumpet... It's the first tutorial book I've ever tried and I find it very useful. For once, I realized that mastering the whole range isn't just about playing a chromatic scales cleanly and hitting that double C (or just about), or practicing only a couple of simple tunes (that's what I was doing before getting the Arban's). Some combinations of intervals are trickier than others simply because I'd never tackled them before... Most importantly, after a couple of weeks using the Arban's method, I'm playing most of the exercises without checking the fingering chart (about time too). Yet, I guess there is a lot of work to do before I can start improvising in the same way I do with the Sax.