Clever way to license/obfuscate web applications until the client has paid for it

Question

I am doing contract work for various clients and I was wondering how best to go about safeguarding myself against non-payment. The projects will be ASP.NET web applications, most likely intranet-based.

I don't have any more details than that. I imagine that the client will want to run and test the applications from their side before making any payment. Is there any way I can "package" the application so that they can't access the source code? Is there an easy way I can make the application break after a certain number of days unless it is "unlocked" by me?