This might be common sense, but if you want to get more battle pets to level 25 faster than the 'normal' pet battling, you can do this by battling the pet masters:
It would be best if you have two level 25 pets already to start with
Step one: find a battle pet master you know you can defeat, maybe one you have defeated previously, but since there's dailies with some of them it means you can do this several times.
Step two: Put in your two level 25 pets in the slots, have a third one between level 20 and 24.
Step three: Go as far in the pet battle as you can with your two level 25 pets. (It doesn't matter if they die)
Step four: Now you have hopefully killed two of the enemy pets, at this point you want to 'pass' on attacking and let your 25 pets die, get the third enemy pet as low hp as you can before doing this.
Step five: Finish off the last enemy pet with your level 20-24 and enjoy the huge amount of pet experience this will grant you (which is for pet masters 1-2 levels)
Hope it's somewhat useful for some of you, enjoy!
Remember this can also be used against non-pet masters but it's not as useful, still good if you're only doing a pet battle every now and then.