Use a brush cutter or a real farm tractor with a mower attachment (rather than a lawn mower)... This will knock very tall grass down to 6 inches or less. Rake out the downed grass, again using a farmer tractor with a side-delivery rake-- or for small areas you can set a lawn mower high as possible, with mulch setting and the grass catcher bag attached-- of course a lawn tractor mower can be used for this step as well. You have to empty the bag very very often and haul a lot of grass off somewhere.
Then as a final step, you can set your lawn mower (or lawn tractor mower) on high cut (3 + inches) and mow it normally to get the grass down to "high normal" lawn length.
Of course I am presuming sort of unlimited capital equipment availability (and skill to use it). If all you have is a 5-7 hp regular lawn mower, the best advice is: Don't ever let your grass get to be a foot tall or more! Failing that, most equipment rental places will have a brush cutter for rent that will knock down tall weeds and grass.