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.

One way to combat an undershoot is to lower the nose and speed up. Especially if the undershoot is caused by headwind or a bunch of sink. For a headwind you need more speed to make progress over the ground. For bad sink you want to fly fast to get out of it. The challenge of speeding up for an overshoot is you end up with a bunch of kinetic energy that has to be disposed of. In short, I agree with Crash one.

On the other hand, I've recently bought and have been flying a SGS1-35C. It is a flapped glider and has no spoilers. The flaps go down to about 80 degrees and cause a boatload of drag. You can really modulate your glideslope by changing speed. If I am high, and not on short final, I can really point the nose at the ground and lose altitude rapidly without a large speed buildup. However, what I am finding is to get back on glideslope, I dive down below it, then as I slow up my performance increases again and I will get back to glideslope. I'll shoot my final approach once I'm stabilized again about 55 mph, though the glider won't stall with full flaps until 35 mph. Makes for nice low energy landings.

Regarding the original question, in either power or glider I prefer to slip on the higher speed side, rather than risk an uncoordinated stall at low altitude. In either a glider or a power plane you can always hold the slip through the flare in ground effect, where you have greater margin over stall. Then again, like will5023 said, maybe it is a problem of perception rather than being at a high AOA.

-- IFMU

In theory, yes, but the effect will be minimal, and the large speed excursions are likely to cause more problems than you solve. As cats_five said, you should be varying your circuit - even to the point of S-turning on final.

I don't know the aero theory for the pitch down moment, but the rearward stick movement in a sideslip is always intended to maintain the pitch angle of the glider - the nose is not raised (unless you already let it fall, and are raising it to the same angle). As noted, the ASI doesn't work properly in a full blooded sideslip. In the abscence of power, pitch sets speed; maintain the constant pitch, you should maintain the same airspeed - therefore you don't stall.

I don't know the aero theory for the pitch down moment

Although a specific Type might have a specific reason .. the general story is

(a) tailplane provides a noseup pitch (ie download) for balance

(b) depending on fuselage airflow in sideslip the lee tailplane surface can see disrupted airflow ("blanketing") and a reduction in downforce overall

(c) result is a nose down pitch

Sometimes this is more evident with flap extended (eg some Cessnas)