My t & c's state that once work halts on a project for two solid weeks due to a lack of feedback from the client, I am allowed to invoice them for the work complete so far.
An invoice can give people a kick up the arse to get their side of the deal done. Failing that, if they pay it then great, you have the cash for your time, move on to the next job (If they don't pay it? Well... you have just unearthed a shitty client it wouldn't have been worth continuing with anyway?)
Make sure they are aware that there is a time issue once a project goes dead and they want to start it up again - probably include that in a letter with said invoice?
Since I've only been freelancing for 3 months this has only happened once so far, and the invoice suceeded in kick-starting the project. Make sure you keep up polite contact with the client, letting people know the invoice is coming because the project is dead, and you need to move onto other deadlines.
So far it's working. I'm thinking two weeks might be too soon, perhaps extend that to 3, or even a month.