As I recall, for every 1000 foot increase in altitude, temperature in F. goes down 5.5 degrees until you reach the dew point, when it then goes down 3.5 degrees per 1000 feet.
This is a rule of thumb and can be affected by other conditions.
To get all technical on you
, these are the "dry adiabatic lapse rate" and "wet adiabatic lapse rate" respectively. They are indeed very useful, but as you say they are rules of thumb. Other conditions (for example an inversion) can cause actual temperatures to vary considerably from these. The difference between the wet and dry lapse rates are the cause behind such phenomena as the "Chinook" or "Foehn" winds (AKA "snoweater wind") which sometimes cause dramatic snow melting on the leeward side of mountainous regions.
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