Only play those who want to play in goalIn youth soccer, everyone is encouraged to try all positions. However, I would not include goalkeeper in that category, at least in game situations. It does no good to put someone in the box who simply does not want to be there. It can be shattering for a youngster to be scored upon. If you must, try everyone in practices, but limit game play to those who want to be (or at least, don't mind being) in net.

Stay positive when goals are scoredIt can be tough to be positive, especially when a simple lapse has just let in the game-losing goal with seconds to play. Goalkeepers need to be able to put mistakes behind them quickly and recover, so try to put a positive spin on things:
Credit the shooter. Sometimes the opposing team has just made a great play; give them credit - it's not always a mistake by the keeper that lets a goal in.

Don't dwell on it. Help the goalkeeper shake it off quickly, make a fast coaching point if necessary, and move on once the kickoff has been taken. You can cover problems in more depth at the next practice.
Look for the technique that needs improvement. Focus on the skill that needs work, not the fact that the goal was allowed. Very often it's just basic footwork or catching that needs brushing up.

Give credit for other than direct savesA keeper can keep the soccer ball out of the net in more ways than making a direct save - forcing a missed shot gets the job done too. Aggressiveness goes a long way here - by making the shooter decide too early, a keeper can force a miss or a pass on an otherwise open shot. A goalkeeper who can get into the heads of opposing forwards early has a huge advantage.

Don't panic after a saveThis applies to both coach and goalkeeper. Give the keeper time to internalize the save (builds confidence), calm down, give field players time to recover, and look upfield to restart the attack. Six seconds is plenty of time to do all this. If you stay calm, you're showing that you have confidence in the keeper's ability to control the ball and the game.

Help the keeper develop good habitsTeach them good technique, then hammer it in with repetition. Don't allow any lazy or sloppy technique, even when they're not officially doing drills or playing. Good habits will show themselves in games just like bad habits. Repetition is the key; doing it right every time (consistency) is what separates a good goalkeeper from a great one.

Don't allow less than full effort, every time, every shotEvery shot deserves an attempt at a save, even if it seems futile. Eventually, the shots that seemed impossible to stop before will start coming into range. If the effort is there, the results will start to come.

If things aren't going well, pull the goalkeeper with careMany coaches will pull a keeper if they are getting shelled, but be aware of the repercussions of this. Some players will take this as a sign you lack confidence in them, and only go downhill. Make sure you stay positive, and point out you're not pulling them because they are a poor player or person, just that it's not their day.