This is what I use for my worm:
http://www.wowhead.com/petcalc#mVdhh0G0bzkM
The reasons for this are as follows:
Spiked Collar, Serpent Swiftness, Culling The Herd, Wild Hunt: These are pure DPS abilities, therefore to make a DPS pet, it only stands to reason to have them!
Charge: This helps the worm get into combat a lot faster, though I find that you may need to use it manually, as when it spits, it tends to use the GCD rather than 'dashing' in first.
Guard Dog, Taunt: Always, *always* keep your growl turned off in instances unless there is an emergency. However, if the tank starts going down, or if an add is pulled, you can activate it and grab and hold threat potentially for long enough to keep someone alive. Even longer if the healer is still available.
Grace Of The Mantis: This is needed to get to Wild Hunt, but it is also extremely useful for the above situation: one crit will spell death for your pet, so crit-immunity will help keep them alive that little extra needed.
Roar Of Sacrifice: Again: needed for Wild Hunt. Honestly, if Wild Hunt wasn't such a good boost for pure attack power, I wouldn't bother with it at all.
Intercept: An add gets free? Or you want to give the tank a little extra help? Another emergency button-push, right here.
Great Stamina: This could just as easily be increased armor. I'm not sure which is more advantageous in an instance. I tend to use higher stamina because armor doesn't help against spells, and the pet is more likely to be targeted by spell effects than physical if it's not emergency-tanking.
As for having enough focus to use 'Burrow', the best thing you can do is to keep 'Bite' turned off auto, and put it on the bar so you can activate it or not. Macro it into a shot or two if you want to use it frequently. However, the pet will not 'save' the focus needed to use a burrow. If the spit isn't needed, it will automatically use 'Bite' whenever it's up. This, especially with Wild Hunt specced, will drain focus very, very fast. So if you know you'll need the AOE? Turn off auto-Bite.
Alternatively, you can pull your pet back to your side, or use /petpassive, to deactivate its attack, and let it regenerate the focus for that moment before you send it back in to unleash. This is hard on the DPS< though. Harder than simply turning Bite off and letting it auto-attack.
Finally, you may want to consider *not* speccing into Wild Hunt. The focus will vanish very quickly thanks to that increased cost. It will lower your pet's overall DPS, but if you're using Burrow instead of physically attacking, it might just be for the better. Turn those three points (2 wild hunt, 1 Roar) into Armor instead.