Install door sweeps or thresholds at the base of all exterior entry doors. While lying on the floor, check for light filtering under doors. This is more effective when performed at night or early morning when it is dark outside and lights are on inside. Pay particular attention to the bottom corners as this is often where rodents and other pests enter. These are common entry points for such pests as rodents, ants, spiders and yellowjackets.
Caulk cracks around windows, doors, fascia boards, etc.
Use a good quality silicone or acrylic latex caulk. Although somewhat less flexible than pure silicone, latex-type caulks clean up easily with water and are paintable. Caulks that dry clear are often easier to use than pigmented caulks since they don't show mistakes. Buy a good caulking gun. Prior to sealing, cracks should be cleaned and any peeling caulk removed to aid adhesion.