Attack them before they attack you. Use dragoons for invasion - they can get back to your base very quickly should trouble arise. Also, keep some dark templars hanging around your base just for safety. Make sure you have lots of detectors in your base as well as some cannons (yes, I said cannons), behind your nex. Cannons can be good if they are placed strategically, for detecting and anti air purposes. Just try not to have huge masses of them - they can be easilly taken out with gaurdians. In later game, when money is nearly infinite, they can be used to occupy center map and defend secondary bases.