Chocolate is good, raisins also work well. They tend to follow the walls so put the trap facing into the wall. Also, sit the trap on a piece of card or something or you might get blood on the floor! In their death throws they can chuck the trap several feet though, they don't always die instantly. If it's a weak trap it may just smash their snout or pin them across the shoulders. You can buy traps which are contained within plastic boxes, kind of sanitizes things a little.
Depending on how the trap is made, some trigger when the food platform is pushed down and others when it is pulled up. Don't ask me why but he ones that pull up to trigger work the best. You can sometimes bend the retaining pin on push-trigger ones into a hook so they work as pull-trigger. Set the trap so that the slightest touch sets it off.
Very soft foods are best avoided as mice are very stealthy critters and can easily pick all of the butter off a trap without setting it off. Use something they'll need to pull at and stick it firmly to the trigger.
Renovated an old house, killed a sh** load of mice! ;0)