Have you tried chocolate ? Cheap, sweet chocolate like galaxy, smeared with a bit of peanut butter. They can smell it a mile off.
I am sure you'll get some other suggestions too, but that works for me and is probably different enough from there normal diet to be interesting.
Another important point is that as 'neophobes' rats generally avoid anything in their environment that's 'new' for quite a while. Get your trap out, leave it and be patient.