How about a sliding scale? Say:
move out by, you get:
21-Sep 2,500
28-Sep 2,200
5-Oct 1,900
12-Oct 1,600
19-Oct 1,300
26-Oct 1,000
2-Nov 700
9-Nov 400
16-Nov 100
Then every week you send them a new letter and cross out the ones that have passed. Nice incentive to move, don't you...