You should use economy picking (hammer on and pull off) when you want to achieve a clean sound with not to much overdrive (Joe Satriani, Vai, etc) and alternate when you want to make a 16 note per-second malmsteen's lick. A tip: if you have a strato, try economy picking with your middle pick-up, it sound quite better. If you have problems with alternate picking, you should try placing your pick in the last fret and gently pressuring it, so you can get more stability. You might want to use your neck pickup, because using the bridge pickup sounds HORRIBLE!!!!!!!!