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I second the 1212... and 2121... practice idea. This stuff should definitely be done with a metronome starting at a very comfortable speed. In addition to focusing on evenness, working on accents helps keep it all locked in and forces you be very conscious of the beat. When I do sixteenth note scales, I do all of the variations of where the beat is relative to the sixteenth note stream.
I developed a reasonably good tremolo on the classical guitar and the way I was taught to do that was similar. I'd do pima, imap, mapi, apim
(p=thumb, a=ring), accenting the first finger in each pattern. Once you get this down, it is easy to remove the accent. | 
