On the 757 go-arounds are as described in the previous post. G/A mode is armed whenever Approach is armed or the flaps are down, then if you want to go around at any time, press the go around switches on the throttles and away it goes.
You have to remember to select LNAV though for it to fly the lateral track as LNAV is not normally used for approaches. If you don't the aircraft will just track straight ahead. This can be at any time after 400ft AGL. Then at some point you'll want to reduce to climb thrust and increase speed, else the G/A will be flown at the speed you at which you commenced the go around. At 1000ft AGL we select Vref +80 kts (about 200kt) and Climb thrust. VNAV can also be used if you have selected a target speed and alltitude on the FMC.
Cheers