Great question. In the current version (1.7.x) there isn't a clear way of doing this. However we are working on this feature for the 2.0.x. For now you have a couple of options that might work.

1. Cross project linking - We have the ability to link an item from one project to another. This process will essentially  pull a requirement from one project to another. It's not a complete copy in that it will still reference the original item while allowing you to modify the new item in the new project. 

2. The hard way - Because the database is open it's possible to run a script that would move an item from one project to another. This is of course, in my opinion, a worse case scenario but it would save you the trouble of having to recreate an item from scratch.

As for the future. We are working on the concept of templates which will enable the user to extract an item, group, or entire project to be reimported as a template. This is kind of like a fancy copy and paste and will allow us to create and share common themes.