It may depend on the species of tree that is causing the problem. I had this problem with Scotch Pine sap, and tried everything. The only thing that worked in my case was alcohol (rubbing or denatured) in a small amount on a rag. No paint damage. It must be noted that this particular car I was working on wasn't mine, and wasn't wax-protected nearly as much as I like to keep my cars. With a more wax-protected car, the other methods recommended by others above probably will work just fine.

Thanks,
Richard