Personally I think IntelliJ gets it right: fully-featured version for 30 days. You shouldn't be any more restrictive than that.
The "fully featured" part is important. If your unregistered version is so crippled as to be useless, how do you expect people to decide to buy it? They have to try features before they decide they like them and want them but the notion some vendors have that people will buy something on the promise of features they haven't been able to try is ludicrous.
Oracle is at the most liberal end of the spectrum. You can basically download a fully-featured version of their software and just use it. They treat licensing as a social problem and there's a lot of merit to that argument.
You should never nag or otherwise annoy your potential customers (beyond possibly expiring the software after a predetermined period). The most notice you should give is "X expires in N days" on say a splash screen or something.
The other thing you can and should do is provide a low barrier to entry for your software.
Provide a cheap (if not free) academic or personal version. Marketing is also a good idea. By this I mean take IntelliJ: it's offered free to approved open source projects.
As for the technical aspects of enforcement: don't be too concerned about the technical aspects. Never choose any scheme that annoys your users or, worse yet, slows down your software (as some schemes do) or (even worse) violates the security of the user's operating system (eg like the Sony rootkit fiasco). Your enforcement is not meant to be hack-proof. If people want to steal your software they will. The enforcement system is there as a gentle reminder (and nothing more) for a social not a technical problem.
