Hi! I have that problem a lot, but I'm long overdue changing the belts. Here's what I do:
* I have a belt tensioner on the belt that slips.
* I make sure the axis are aligned properly
* I use machine oil on the rods... both x and y at the head and the ones along the sides where the belts are attached.
* Before each run, I keep the printer shut off and move the head around by hand.
Normally, I'll have problems if the UM1 sits around a week or more between runs. When I'm running ABS, because the inside temps are hotter and I'm using sewing machine oil on the rods, I find i have to lubricate them more often.
Good luck!