When jigsawing, make sure you can turn the speed right down. I've tried to do it quickly, and ended up having to do it all again, as the plastic melts and then joins it's self back together behind the blade.
Just go as slow as you can and check that the cut line hasn't sealed up.
Depending on what you're cutting, it might be worth wedging something in behind the saw to keep the two pieces apart after they've been cut.
HTH
J