Just a short thread as it has already been said elsewhere, but do not use the' pet attack' button from the pet window (except when you are casting, as this is the only way to tell your pet to attack while you are doing something else).
Make yourself a macro which clears the pet's hatelist right before an attack command is issued. This way, you will have a much better control over what target your pet is attacking, long before the pet hold line of AAs will allow you to use /pet focus. Especially when fighting multiples, this command will work almost every time when you want to switch targets - again, long before /pet focus comes live. In raids, it will also make sure your pet doesn't rush the wrong mob, you'll see how useful this macro will be in Velious era in Temple of Veeshan (aoes everywhere... but your macro will make sure the pet sticks on what you are told to !)
Line 1 /pet back off (backing pet off = clearing its hate list)
Line 2 /pet attack (no /pause needed)
You will thank the Gods for that macro, believe me.