Good question. Google usually has good answers and here's one:

http://www.mcfedries.com/JavaScript/mandatory.asp

I imagine that one really easy way of doing this is by making them pay for the service (done thru admin panel). If it was me, and this was a free service, and you wanted that kind of information mandatory from me, I would either put false information or go elsewhere. Chances are your members will do same by giving false information thereby rendering the "mandatory" fields as meaningless.