You can't prevent people from cracking your software.
However, you can make them create cracks that will hurt your sales less. Keygenerators that can issue a valid registration code for your software are much worse than simple patches that remove registration incentives from your software. That's because a crack will work for one software version only, and will cease to work with the next software update you release. The keygenerator will continue to work until you change your registration key algorithm and that's something you don't want to do often because it will put off your honest clients.
So, if you are looking for a method to fight illegal keygenerators for your software and you do not want to use assymetric encryption because of the long registration codes this generates, you might have a look at Partial Key Verification.
Partial Key Verification makes sure that each illegal keygenerator works only for one particular release of your software. Basically what you do is to make sure that each release of your software only links with the code for checking SOME digits of the registration code. Which digits exactly is random, so crackers would have to reverse engineer many different versions of your software and combine all this into one keygenerator in order to release a keygenerator that works for all versions of your software.
If you release new software versions on a regular basis, this leads to numerous keygenerators spread on all kinds of software piracy archives which are not working anymore. Potential software pirates usually look for a crack or keygen for the latest version, so they will likely try a few of those and give up eventually.
I've used the Partial Key Verification in my (C++) newer shareware games and it has been very effective. Before we had plenty of problems with keygenerators which we could not fight. Afterewards there were lots of cracks and some few keygenerators that worked only for that particular version of the game, but no key generator that would work with all versions. We regularly released very minor updates of the game and to render all previously existing cracks useless.
There seems to be an open source .NET framework for Partial Key Verification, although I have not tried it.