Since life has a way of intruding into a one-sitting script read for me as well (usually in the form of a diaper change, another bottle, or "Dad, Jane is looking at me and won't stop!!!"), I also need to employ little tricks to make a screenplay reading as fruitful as possible.

I try to read in acts. Act 1 in one sitting, first half of Act 2, second half of Act 2, and then polish off Act 3 when the kids are in bed. Even if I don't have a clear idea where the act breaks are, I'll usually just take a coffee/Oreo break at the 30-60-90 marks.