As stated...
write, then write, then write some more, then when you get tired of that write.
Tip:
DO NOT REWRITE A SINGLE WORD UNTIL THE ENTIRE SCRIPT IS COMPLETED.
That is the #1 mistake, getting bogged down rewriting a scene over and over. Write the whole script, don't even reread what you just wrote, just forge ahead, THEN come back and rewrite the whole thing, then rewrite it again, the rewrite it again, then rewrite it again. Once you get the hang of it you may only have to rewrite a script 6 or 8 times before it's pretty good.