ALWAYS get lock to summon you to the instance, then get a boost from lvl 70 with very good gear (S3+ or T5 +), when you get to 50 you may need a help from another 70 I guess.
also if you take quests for the instances that could help too.
palas/warrs/hunts are the best boosters IMO, palas got great survi and never ending concentration and warrs got good survi too and can thunder clap+cleave+demo shout till death, and hunters just dont need to make any breaks whilst boosting i guess.
get some lowbies in your group, you will get more xp from that (credits to [Eski] for that one)
instances you should do:
1-10 level like you would normally
15-20 Stockades/ SFK if you are Horde and CBA to get a lock to summon you there.
20-27 Gnomeregan.
27-35 SM Cathedral (you CAN enter Cath from 27 did it with my mage)
35-40 Zul'Farrak, idk if you can enter it from 35 but probably
40-50 Blackrock Depths
50-55 Lower Blackrock Spire.
55-60 Hellfire Ramparts.
60-70 Questing in outlands ofc
if anything is wrong dont blame me this is just a guess![]()