I just read a good article from First Round on this: http://firstround.com/article/Looking-for-Love-in-All-The-Wrong-Places-How-to-Find-a-Co-Founder. They recommend a minimum 30-day dating period and suggest that parties "agree on whatever terms make sense - you both get to keep the code, for example..." Of course, "code" could be replaced by something else such as prototypes, customer lists, marketing materials.
It can be tricky to set-up a way to split equity during this time. If you haven't incorporated yet, ownership stakes could become unclear if one person pursues it and the other does not. A simpler way would be to set out a specific project that both can own, or, as Ian suggests above, a project that is not critical to the business.
If you've already been working on something for a long time and you would still like to own the IP, then you could pay the person you're "dating" an hourly or total amount that both agree to. Or you could set up a clear timeline and total amount of time you both will work, in hours or days or weeks, by which time you'll both come to a decision on whether to work together. Set up clear metrics upon which you'll evaluate each other. Some of it will be subjective, but that can help to minimize the feeling that one's efforts are being unfairly valued.
This is a common question that I've faced so I'm eager to hear other peoples' thoughts as well.