I ask for one month's deposit for retainer clients in addition to the pre-paid retainer payment. I hold the deposits until the end of the contract term and apply that to the last month. I've tried using the deposit toward ongoing work but that becomes 'nightmare-ish accounting' for me. I have a 30-day notification of the termination of a contract so this has worked out well and then I just apply that deposit to the last month's work.
For hourly clients, which I have not had lately, I required a minimum of two hours per week so the hourly deposit is 8 hours minimum. Again, I require 30 day's notice so the deposit is applied to that last month.
I haven't done any small assignments lately because I rarely do any project work but when I have I ask for two hours deposit because even with project work I still have a two hour minimum.