Install a solid-core door
Add a door sweep at the bottom
Apply weatherstripping around the frame
Seal gaps with acoustic caulk
Use a door gasket kit
Hang a heavy soundproof blanket over the door
Add mass-loaded vinyl to the door surface
Install an automatic drop seal
Replace hollow-core doors
Tighten or replace loose hinges
Fill gaps around the frame with backer rod and sealant
Use a door threshold seal
Add a second door if possible
Cover the door with dense panels or acoustic panels
Ensure the latch pulls the door tightly closed