Android : Implement Free Versions Of Applications?

I already mentioned this briefly in another post as part of another publishing topic.

But I think it is pretty important for a lot of us to know good ways to make free versions that can't be cracked too easily etc. After all most people just download the free stuff, I know I do, so its important for numbers.

I just made a game called ScrollSlider and I want to make a free version with limited evaluation period perhaps, but I could use some advice on how to do it. I'm also having problems with data persistence as views appear and disappear and managing game loop view thread accordingly.