Try the ‘Ropebomb’ for slab rappels

On a lower angle rappel descent, such a slab, tossing the rope in the traditional manner with a butterfly coil typically leads to a tangled mess halfway down the rap.  This leaves the first person rappelling the unpleasant and slightly risky task of declustering the spaghetti pile before proceeding.

Rather then the butterfly coil, try the “rope bomb”.  The round nature of this “coil” can allow the rope to actually roll down a slab, hopefully giving you a perfect rope toss and no detangling required.

After threading your rap anchor and finding the middle of the rope (see this tip for a crafty way to do this) hold the ends of the rope in one fist.
Then, simply wrap the two strands of rope around your fist, creating something that looks like the photo below.
Your mileage may vary on this tip, so try it in a more controlled setting before an actual climb.

This tip and photo are from “The Complete Guide to Climbing and Mountaineering”, by Pete Hill