Wow; everyone's posting their cents load; I guess I'll put up some of mine as well, just some opinions based on the viewpoints of a GK:
1. Defenders should never stand in front of the person they're marking. Always stand 1-2 steps behind them or 1 step either left or right of them. Standing in front of them will give them more time to react if the ball cuts in behind you. This is the same for GKs.
2. Set-piece defending. Most set piece defending are won by i)positioning and ii) anticipation. Always start your first defensive position to cut out all passing options to the opposing team. Next; be aware of the players running into the box and anticipate the ball's movement when released to them. When you're part of a defensive wall; NEVER turn your back to the ball; just cover your vital parts with your hands. If you're scared of the ball, you should not even be playing. This is equally true when blocking a striker's shot. Take it like a man.
3. 1-on-1 and 2-on-2 defending. If your team has conceeded a breakaway; for a 1-on-1, let the GK deal with the oncoming striker; your job is to cover the GK's loose angles and deal with any loose balls. You should head towards the goal line, covering any available angles. It has to be remembered that in 1-on-1 and 2-on-2 situations; the best form of defense is time wasting. You're trying to stall the opponents while your teammates return to help out in defending. For 2-on-2s; once again, depending on how far into your area the opponents are; remember to utilize your GK whenever possible. If the 2 strikers are onto you and your GK is behind you; stand central to the goalposts, outside the D box to push the opponents into separate wings to reduce their shooting angles; and in that central location; just block all attempts on goal while your teammates return. However; if your GK is not behind you; ensure that you take on one man while the GK handles the other. In most cases; take the nearest opponent.
4. If you know you can't get to the ball; let it go. Trying to be too gung ho will only result in injuries.
5. Your GK is also playing and can help serve as the extra man where possible. However, be careful not to place the GK under unnecessary pressure to avoid mistakes.
Of all the positions in Futsal; it's ironic that GK is the hardest and most crucial; yet, it's often the most neglected part of the team. Ask any international futsal coach and he'll tell you that finding a GK is the hardest part of team selection. This is mainly because futsal GKs cover alot of technical and physical conditioning and training as they have to do 2 roles, whereas outfield players only have 1 main role.