I think its not importent what protector you are taking. Its the features that makes your application unique. When your app is good enough, people will pay for it. No application is uncrackable. Every time when an application is cracked it means that your application is spread a little wider and more people are seeing it. I've seen some superb shareware software in my life, but no cracks for this. Most people don't know about them.
When the app is cool. Some people will pay for it. There will be everytime where some are not paying.

Change the checking for registration in your app regulary and add some randomizing to it.

Cheers, neogen