Dug this old one up on the internet.

TILD: Thirst Induced Lucid Dream

This technique works by cutting out everything called drinking for so
long that you can really feel that you are thirsty. Go to bed as
normal and sleep off that deep delta sleep, wake up after 5-6 hours
and go to the bathroom. Fill a glass with tempting, fresh cold
water. Try to think of water and doing a RT when you drink. To make
the experiment more successful you should fill your mouth with salt so
that you really feel like you need to drink something. Set the glass
of water on your night table and while you fall asleep, think on
drinking the water and at the same time doing a Reality Test. You will
not drink anything in the real world, so if you find yourself drinking
something, then it must be a dream. If you want to increase the
feeling of thirst even more, you should try eating some pepper or
chili before going to sleep again.