Take, say, 100 10-second exposures at high sensitivity, find the 2-d cross-correlation function between each pair of consecutive images (assuming you have nothing else in the FOV, or if so then mask it out), and shift and co-add them? If the noise is additive and normally distributed you can continue ad nauseum–the SNR is proportional to sqrt(N) where N is the number of integrations.
(comment by "Ben" on http://blog.jonathangazeley.com/2010/09/taking-pictures-of-the-night-sky/)