Take a look at the article Developing for Software Protection and Licensing; it explains how to choose a solution, why you should obfuscate your application and gives a number of tips for structuring your code to be harder to crack.
The thing to remember is that once your application is running on another machine, there is no 100% guaranteed way to prevent execution. What you can do, if you're careful, is raise the bar sufficiently high that it's easier to purchase your software than to crack it.
Obligatory disclaimer & plug: the company I co-founded produces the OffByZero Cobalt software licensing solution for .NET, which supports VB.NET natively.
