Cracking the brakes works a charm. Also works when you are in straight and level - ie aerotow retrieve - and stop paying attention.
Going into low tow can also help for long periods of straight and level, and keeping a bit of forward trim.
Be very careful when you do crack the brakes though - especially if your gliders brakes have a tendency to suck out. Also worth mentioning that in the descent you may need to remind the tug pilot of your maximum aerotow speed.
I'd be interested to know why you were descending at 750ft on tow though? The only time I've had to descend on tow was because we were at cloud base and the guy on the back chose to hang on...no, I don't know either.