For my committee recommendation, me and a bunch of my pre-health profession friends that also applied for the letter went to each of the committee members and rushed them.
We must've done something right b/c they moved up the meeting to vote for the letter by 3 months. Ended up getting it submitted yesterday.
So definitely show how much you need and want the letter. Not only that, explain to him/her why you need it bad and ASAP! haha GL! If the professor is at your school in the summer, approach him/her after class.
Ray