One of my favorite licensing schemes like this that I've seen (keeping in  
mind I'm not a big fan of this sort of thing in the first place :) ) is  
one in which a license key is encrypted with information that includes  
whether there's an expiration date and if so when it is.  The user enters  
this key (or you could have them download it, or whatever) and then the  
application decrypts the key to determine whether the application is still  
licensed to run.

The reason I like it is that it's sophisticated enough to thwart naive  
attacks like manually resetting a timestamp in the registry, and yet as  
long as the user keeps a copy of the key somewhere (a good reason to keep  
it short enough to write down) they will always be able to use their  
software, and it's also weak enough that a user can work around a  
licensing implementation error simply by resetting the clock on the  
computer (it's important for any copy protection scheme to avoid  
inconveniencing legitimate users, especially in ways that lock them out of  
the application altogether).

The above scheme relies on you generating and transmitting the key to the  
user, but you could implement a variation on the scheme in which the  
program itself generates the key.  This is less secure, since all the user  
has to do is remove the application and any associated data in the  
registry, but frankly if you've got a user who knows enough about  
licensing implementations and the registry to hack that, they probably  
know enough to get around any reasonably simple licensing implementation.  
And since it's a bad idea to implement really obnoxious schemes that would  
be more resistant to attack, because of the potential for inconveniencing  
your legitimate users, you might as well accept the possibility of those  
few more-sophisticated users getting around the scheme.

All that said, IMHO the best licensing scheme is none at all.  Instead,  
make sure you're providing some added value in the form of customer  
support, regular updates, user forums, etc. that makes users want to pay  
you for the product even if they can pirate it.

Pete