1) Build your own PTs...don't sit around waiting for an invite.
2) Try to build PTs around a BRD and/or a COR. These jobs exist in large part to facilitate faster XPing.
3) Try to match damage types to the weaknesses of the monsters you are fighting. Perhaps the best known example of this is the fact that all flying creatures are weak to piercing.
4) Take advantage of Fields of Valor...it's free XP.
You will never level as fast as you can in WoW, but these steps will at least make things go somewhat faster.
5) Level support jobs that are in demand, and very useful in endgame. RDM and BRD for example are fairly cheap, and always in demand, and useful in every event/pt. After 50-55 every pt would ideally like to have one. Have your sub jobs leveled to keep your character appealing or desirable I.E. /whm /sch /blm for mages /nin /sam as DD's
6) Get in as many parties as possible. You'll find some very odd combination of jobs, mixed mage/ DD jobs /ninja make for very efficient experience point gains. Example : PLD tanks with DD's that rely on MP make BRD, COR incredibly efficient, and keep them quite sane while they're buffing your PT. PLD,COR, BLU, DRK, BRD all work well in PTs that rely heavily on MP, with one main heal. I.E. RDM, SCH, WHM.
Absoplutely the best advice, but this will be much faster. (good luck getting the ppl tho) ~SMN BURN~ grab a 75 friend, pld or anything with rdm sub, a level sync person lvl 12-16ish and 4 smn and go to korroloka tunnel, have the 75 grab a BST giant and CFH on it and link a bunch of bats, as many as possible, bring them to the camp, and have the summoners AF the CFH giant, make sure they have ethers cuz they will need to AF more than once. you will kill all the bats and make around 40k exp in about an hours worth of work. if you have cor or know some cors you can try to get the snm 2h back and do it again (will need a new lvl sync person tho since he should be around lvl 21 now lol)