Apart from the necessary task of revision etc, some other things I find useful to improve the work:

1. Reading more poetry - I attempt to find and read one new poet a week.
2. Memorizing poetry - "the music of what happens" (as Seamus Heaney called it) is better heard when engraved in one's head.
3. Reading, or attempting to read, books on craft (forms, prosody,history,ideas to jump start writing etc) and criticisim, along with essays on related subjects (about poets, aesthetics, history etc)
4. Putting aside a day in the week to the exclusive task of reading and writing.
5. Walking around.
6. Updating lists of subjects I want to write about/on, but for which I don't have the words yet.
7. Finally getting hard hitting or what I term "slash & burn" criticism (The Chinese say, to become a master, one has to eat bitter. Also more recently there was this evocative passage in Marquez's memoir about the music he faced when he unveiled his first short story - his critic friend burnt it in an ash tray. How nice!) is always a good idea.

Enjoy writing.