1 nice turret at the entrance would also help as they HAVE to come from somewhere (or 2 for ur expand)

unless its a drop, then 2 in ur base too if ur continuously getting infestor harassed

There are so much timing to remember if I'm playing mech
i) Roach rush timing
ii) Mutalisk timing
iii) BL timing
iv) Ultralisk timing
v) Roach switch (good against heavy thor n hellion composition) / Zergling switch (good against heavy thor n tank) / Mass Muta switch (good against heavy tank n hellion composition)
V) BL / Ultra switch

which style do u play more often?
Dont try to rely on that too often, when u get used to it, you don't know what to do next if your attack fail.