My contracts are usually to design and develop new product for client. It is rather longer term involvement and I just bill them for 8 hours per day disregarding on what happens on any particular day. Clients know about it and it is agreed upon. If they are not happy about my work which luckily had never happened they're free to show me the door.

Every once in a while I also do short term contracts with very limited scope. For those I would just do a lump sum payable in stages or just initial deposit and then pay balance at the end.