ok, here's my 2 cents.

pylons at edges of base to spot - Nydus wryms and Medivac drops

or

1 cannon - to ward off / buy time for reinforcements

or

if u have high templars, keep 1 or 2 - very effective against muta harrases and MM

or

scout early with phoneixes / hallucinated phoneixes

or

my personal favourite - pylon at mineral line, warp in reinforcements immediately once you see drops. simple, practical and effective.