Up till now the conventional answer was: "suffer." Low level pets only gain experience when you kill something around your level range, not theirs, so the only choice is to just bear with it for a very long time while you try to kill high-level monsters with a low-level pet, or else just give up and not do it.
Well finally it looks like Blizzard has in mind something to do about it. Drysc tells us:
We have plans to adjust how quickly a hunter's pet will level if there's a large gap between it and the hunter's level. However, it's still just a plan and not something we've implemented and thus are not ready to discuss it.Something's going to happen -- we just don't know what it is yet. But really, for hunters anything would be better than what it is now. It's always nice to remove needless tedium don't you think?