What you want to do is harass, so plan for it! Here are some examples:
- Mech? Hellion drop or driveby.
- Bio? Early 2-rax pressure, or marine drops
- Heavy air? Banshee harass.
- Lategame? Nukes or Ghost drop with snipes.
- Twilight Council tech path? Dark Templar.
- Robo first? Try some warp prism drops or warp-ins.
- Muta harass (typical)
- Baneling drops on mineral lines (my new favourite)
- Zergling run-bys or counterattacks whenever he moves out
- Infestor drops
- Burrowed Roaches or Infestors
The most common mistake to make when figuring out how to add harassment to a build is thinking that you have to add a lot to your build. Try adding as little as possible. If going heavy Roach, add Tunneling Claws and no more. If you're going Mech, don't try to marine-drop - just use Hellions as obnoxiously as possible. If you're doing a 3-gate robo, don't feel that you have to add DTs - just drop or warp some Zealots.