- mix 1-2 ml of an o/n culture (in 2YT, LB, whatever) with 1/4 vol.80% glycerol (100% glycerol is impossible to pipet)
- dip tube in liquid nitrogen or -80 C ethanol (no nitrogen or ethanol around? just bang the tubes straight into the freezer)
- store at -80 C

7% DMSO instead of glycerol also works

instead of an o/n culture, you can scrape cells from a plate, put tem in 2YT, LB, whatever, and make a stock