I have used three methods with some success...

1. Layer it between pieces of paper and cut with sharp scissors or exacto blade, the paper helps keep the latex from stretching and moving about.

2. Roll the latex into a tight tube, cut strips by slicing cross sections with sharp razor blade. The tighter the roll the easier it is to cut

3. Use a rotary cutter. These are rotating disk blades used for cutting cloth, look for them wherever quilting supplies are sold. I'm lucky enough to be able to borrow one from my wife.