Everyone has advice about how to move a baby out of the "family bed," just go in and comfort them and don't pick them up again, and so on. However, our 7-month-old son shares a room with his 2-year-old sister (we're trying to help him spend the night in his crib). Any hints on how to get through several really awful nights?
It seems like your question isn't what to do—retrain your son to get to sleep, and be satisfied with sleeping, in his own space—but how to do it.
The first and most important step is to steel your resolve and stick with a consistent routine. Inconsistency teaches your son only that you will break down if he persists hard enough.
How to do this? Many approaches work. The right one depends upon your comfort level with letting him fuss for a bit. There are several popular methods. (You can learn about these different methods by reading A Review of the Sleep Experts.
Whatever method you choose, be prepared for the transition to better sleeping to take at least a few nights. You may want to consider moving your daughter to a different room during this time. You could turn this "trial" into an adventure! Perhaps your daughter can "camp out" with her dad in a part of the house away from the noise while you tend to your young son.
Remember to be strong and consistent in whatever approach you choose. Most infants can be successfully retrained in about a week.