1) pull out full airbrakes (they are very effective in modern gliders)
2) side slip if airbrakes are not getting you down enough. Just be careful - some gliders have a horrid habit of dropping the nose in a side slip with airbrakes. You need to know what the one you are flying does.
3) if you still land too far down a small field, ground-loop the glider. It might damage it but that's better than damaging the pilot.
In my experience if you spot you are too high early enough to try to burn off height by increasing speed you are far enough away from actually landing to widen the circuit, and/or pull the brakes. You certainly don't want to land too fast in a small field - or too slow in any field.