What to Bait a Rat Trap With?

I have rats or mice at the moment that are going through my veggie patch and eating all my butternuts and the odd tomato. I have tried baiting a rat trap with carrots, tomatoes, meat, even pumpkin, but I cannot catch the little blighters. They would much rather eat my butternuts.

Can someone please tell what to bait the trap with that actually works? I have heard so many stories but really want a tried and trusted method as I am really quite desperate.