Watch me get 15 levels in one rest! Just save up your EXP (don't rest) untill you no longer have hunts or sidequests that you can complete with ease. Then, make sure you have $30k gil saved up and head for The Leville (the person at the right side counter) in Past Altissia. Bingo! 3.0x multiplier bonus!
Also, consider eating the fat chocobo burger at Chocobo Wiz to get an additional 50% EXP bonus from battles!
