There's some really excellent piranha tactica on tau-online, but in the meantime, here's some advice:
1: Don't load up on gear. They're fragile and expensive enough as it is. Targeting arrays if you have a fusion gun, maybe decoys if you can afford it.
2: Units of 1 or 2. Too many together are too hard to hide behind terrain. And more fast-moving scoring units the better.
3: Fusion Blaster is your friend. Buy it and go run up and break tanks.
4: Terrain is your friend. Hide behind it whenever you can.
5: Speed is your friend. Always, always ALWAYS be moving enough to get your 'speeders moving fast' rule.
6: Don't forget your drones. Launch them to tangle up enemy units until real help arrives (great for bogging down a small heavy weapon team). You're open-topped and drones are I4. Zoom in, drop drones and have them charge. Drones are also a good free fire-screen for ICs. They force an enemy squad to overkill a pair of free drones if they even want a shot at the IC hiding behind them.
Here's my usual setup for piranha.
1 squad of 2 piranha, Fusion, TA
1 squad with 1 piranha w/2 seeker missiles.
The squad of two flies around and blows up tanks and steals objectives. The solo unit keeps its fragile, expensive, ass back out of the way. It provides seeker support and gives my battlesuit commander a drone screen. Turn 5 or 6 it might break from cover and make a run for uncontested objectives. Otherwise he flies back and fourth through my deployment zone trying not to get shot down.
With the burst cannon and drones it's tempting to mass up a good sized squad and just strafe the holy hell out of infantry, but I'd rather charge a tank than a unit of S4+ guns. Against IG, fine, strafe away, but against most other races with decent guns you're going to get shot to shit. A tank only has a couple guns it can bring to bear, but ten rapid-firing boltguns can be potentially more dangerous. Especially with their own special and heavy weapons thrown in. So take the heat gun and go tank hunting.