For the six month anniversary event everyone got vials that give you 100% xp boost for 2 hours. The name of the vial is "Veterans Experience Vial", it came in the mail and you should have gotten 10 of them. There is another one called Heroic Experience Vial that increases your xp by 50%. The two vials do not stack however, so only pop one or the other do not try to do both - the latest you drink will overwrite any older ones.
Patch 1.5 introduces AAs, or Alternative Advances, for level 50s that let you increase stats and abilities slowly, converting XP (normally wasted at 50) into AA points. You need to earn 500k XP to ding a "level" that gets you 100 points to spend on AAs.
So we combine the two to get some boosting. You access the first chronicle, celebration in Meridian or Sanctum, by pressing the J key (at level 50, can't do it before). Enter it, and then pop your veteran XP vial. Finish the chronicle, and at the end instead of just getting a auto 500k xp boost (for your first AA point) you will receive double, or 1 million XP and basically an extra AA level.
You can only do this the first time, if you already have done your attunement chronicle you can't get double XP the second time. The vials do work in the Greenscale and Hammerknell chronicle though and you will get double the normal XP.