Sewing leather

Has anyone on my flist ever used a sewing machine on leather? If so, do you have any tips? Kidunit is making a leather vest (or, well, trying to) and we're having a devil of a time with it, even though the machine in question (a classroom-style Necci) is supposed to be able to sew on leather. However it keeps bunching up, and also the stitch length gets weirdly minuscule. I'd heard that if you put some tissue paper down it helps but all that seems to do is get tissue paper caught in the stitches and it's a bitch to get out.