I've started having success using cheap harass. Like Day9 said, mobility is mech's weakness, but if you spend all your gas on mutas, your great harass will be worthing nothing when terrant decides it's time to fight army vs army.

However, 2 infestors in an overlord cost something like 500 gas, and they do HUGELY more damage than 5 mutas, while using less than half the control. In fact, up it to 4 infestors in an overlord (same suppy as 2 swarmhosts), and you can pretty much blow nuke-sized holes in the T base every time he moves to the bottom of his natural ramp. Killing armories and factories where I can't kill SCVs, this gives me quite a macro advantage off the back of 3 base play. You can do this while building up a roach-hydra-viper force, and then even if the terran gets to 200/200 in a decent time, you can still have an alright-ish engagement with him when your armies bash into one another. Meanwhile your infestors just took out 2 factories in his main, and you're remaxing with ultras or more roach hydra viper, while the remains of his mech force try to decide whether to save their base, or attack into yours without any reinforcements.

In short, it's like Macismyname said - harass like mad. Just be aware of how stupidy good infestors are for harassing without using much supply. If the terran has tanks on defense, overlord drop and burrow still make it absolutely trivial to spawn infested terrans and kill the tanks, and when the tanks are dead, his base is pretty much open to abuse. Also note that missile turrets do pretty much nothing to defend against it, especially if you have a spotter (I use an overseer and changelings to spot before dropping).

TLDR: Go roach+hydra+viper, and use 2-4 infestors drop harassing to destroy his base whenever he leaves it.