How do I clean behind the trackpad on a late 2010 MacBook Pro?
The tolerances in the unibody macs are very, very tight and the area is located behind the battery, so Apple does not recommend users without training on servicing the delicate back sides of the battery clean it other than spraying air lightly from the outside. I've found setting it up vertically and open helps with gravity bringing the grit towards the most open side of the trackpad. (The part closest to you that swings free and presses on the physical spring.)

It's not hard to find the tools or the steps - iFixit.com has both, but I prefer to take it in to the genius bar for cleaning as long as it's a convenient trip. If you're careful, you should be able to get inside the mac and see how much debris is inside and whether it's worth a trip to the shop or to learn how to clean it better yourself. Getting the back off is all you need following the memory replacement instructions to have a peek.