Altama: Tactical boots are similar to street shoe size. Desert, Jungle, and Combat boots are slightly larger in fit than normal size.
Belleville Unlined (non-waterproof) boots: Select a full size smaller than your dress shoe size. For instance, if you normally wear a 9.5 Regular in a dress shoe, select 8.5 and it should fit.
Belleville Waterproof boots: Select one-half size smaller than your dress shoe size. For instance, if you normally wear a 9.5 Regular in a dress shoe, select 9.0 and it should fit.
Danner USMC boots: Select a size one-half larger than your dress shoe size. Danner USMC boots are cut smaller than regular Danner boots.
Bates USMC Lights: Select a size one-half smaller than your dress shoe size. They used to fit true-to-size, but something changed in the production line and they fit differently now.
Everyone else: All other brands seem to be "true-to-size" - meaning they correspond to what your foot would measure and how it would fit in a dress shoe.
In general, boots with additional lining such as Gore-Tex or Thinsulate fit more snugly than un-lined boots.