I get double vision sometimes when I'm very tired. What can I do?

See an eye doctor. The brain keeps the two eyes working together so that each eye is looking at the same object. When some people get very tired, the control over the two eyes can temporarily slip, causing double vision. The usual treatment for this is rest and sleep. If this is new, or happening frequently, you should have a general eye examination.
Pencil Pushups. Double vision when tired may be the result of a muscle imbalance, similar to a lazy eye in children. Pencil pushups may be tried to strenthen the muscle, where you look at print on the end of a pencil held at 12 inches for 15 seconds then look at a distance and repeat. If this does not help, special glasses with prism may be necessary.