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The best plan is to select the best bait and appropriately sized humane trap. But to do that you must identify the intruders. Are they squirrels, or raccoons? Additionally, before you trap them, you need to seal off their entrances into the attic so more don't come in after you get rid of the ones you have.
Raccoons are nasty dangerous critters so I'm hoping what you have is squirrels.
Best bait for squirrels, I've found, is to grind some almonds or pecans and some raw shelled sunflower in the cuisinart, then mix it with more or less equal parts of peanut oil and honey.
For raccoons, they seem to love granola cereal topped with honey and raw shelled sunflower seeds.
Orchard Supply Hardware (owned by SKmart) has both types of traps.