Step 1: Hotkey units on 1-9
Step 2: Slam face on keyboard numpad
Holding down a hotkey to build things (even if you don't have the minerals) also increases your APM.
Lots of players (especially the pros) will spam actions in the quieter parts of the early game to keep their fingers warmed up, or to start building a unit the millisecond they have enough minerals to afford it.
It's not unreasonable to have 200+ real APM in bursts, though. For example, at the beginning of games in Brood War, pretty much anyone competent is going to do something along the lines of the following in about 3 seconds:
click command center
build scv
select all scvs
mine a mineral patch
select a scv
mine a different mineral patch
select a scv
mine a different mineral patch
select a scv
mine a different mineral patch
click command center
set command center hotkey
set command center rally point
set camera hotkey
which is 280 APM. You'll see similar bursts during macro cycles, which are generally just (click building, press M) repeated 10 or so times in a short period.