Airport bus link is running between both airports from 5am till Midnight every day.
To go from Suvarnabhumi (Main Bangkok Airport)Don Mueang: get on the Shuttle bus at Level 2 between gates 2 and 3
To go from Don Mueang - Suvarnabhumi: take the shuttle bus at the ground floor arrival of terminal 1. Departure from Don Mueang is outside Gate 5 it is easy to find with a blue counter nearby.