Give yourself plenty of time to complete an assignment.
Write and erase possible ideas in your head. At some point, one idea will keep returning.
Read over your notes, outlines, etc. Then immediately write whatever comes to mind.
Pretend you are writing to a friend about your topic.
Start in the middle.
Open the dictionary, point to a word at random, and use it in a sentence. Keep writing.
Write nonsense until something makes sense.
Go away for a while. Write later.