There are not many, if any, truly simple tremolo pieces around, that I could find anyway. I've found it best to pick a simple arpeggio piece and modify it to tremolo. So that I can concentrate on the tremolo and not worry about my left hand too much.
I'm not a huge fan of the Carcassi piece op60#7 for tremolo. While it is an interesting piece I dont feel it's done much to help my tremolo.
I've started practicing a tremolo version of Tarrega's Sudy in E minor as done by macdupac
in the mp3 section viewtopic.php?t=2504
. This is really helping me for 2 reasons. 1- It is quite beautiful to play (with or without tremolo) and 2 - The tremolo is all on the 1st string which is easier.
Also a small change in the way I hold the guitar has really squared up my hands and made it much easier to play tremolo. The idea for this came from reading this posting: viewtopic.php?t=2008
Finally another article I read about practicing scales using tremolo, PAMI, is also helping. I try and play each note for as long as it takes to sound right and then move on.