1. Learn endurance training in the beast mastery tree.
2. Go to the hunter trainer. (make sure you have your pet out!)
3. Reset your talents.
4. Repeat steps 1 - 3 until you have as much hp as you want.
5. Go to outlands or something and have your pet (and only your pet) attack a mob. Walk in the opposite direction of your pet until it gets dismissed.
6. Summon your pet. Your pet now has a lot of health.
7. Additionally, you can learn spirit bond, which regenerates %2 of your pets health to you.
Which is a lot.