How to get XP fast

So you want to powerlevel? Well, sorry there's no 'ultimate' way. It all depends on your archetype and your skills, but here's a few tips.

Ghosting paper missions (Stalkers delight)
One way of getting xp fast is to learn how to ghost missions. Ghosting is the art of running/jumping to the end of the mission, click the glowie and/or defeat the boss and his/her minions. That way you get the mission bonus. Ergo... fast xp. Anyone can actually do this, but if you lack the skills here's a few pointers to make it easier:

1. Play a stalker - they're natural ghosters and they also suck, a lot!
2. Get SuperSpeed as travel power - it's a stealth power too (handy in pvp too even for stalkers)
3. Get some other stealth power - several ATs has some kind of stealth power, and you can get a stealth recipe for all travel powers.
4. Jump, jump, jump - somehow the baddies have a problem spotting you if you jump past them.
5. If you get spotted, keep running/jumping since the AI is so stupid they won't follow you for long. Thank god for that.

Now, ghosting paper missions are pretty boring, trust me.

Hunting - old style XP farming (good for damage doers)
Hunting in the streets used to be a powerlevellers #1 way of solo-levelling. Hunt higher level minions or bosses if you can, and get nice xp. Very bad idea according to me, since if you die you get full debt instead of mission ½ debt. Also quite boring, but a favourite among some so called "powerteams".

8 man teams on relentless (great for squishys)
My personal favourite, and a good way to make XP if you're a bit squishy. My alts are all geared toward teamplay, since thats what MMO's are all about according to me. A good full team can make loads of xp very fast on a higher diff setting, a bad team can get some respectable debt. ...but it's always fun! This is really what the game is about IMHO.

PvP glory (all non-stalkers, and perhaps a few stalkers too?)
The ultimate way of making huge XP/Inf/prestige is - for many alts - the PvP zones. Stay clear of real PvP though. As soon as you're 'old' enough for Bloody Bay, go there and do the missions over and over again. Unfortunately the best mission is 'bomb the base' and requires you to 'kill-all', but it's great xp. Get a team of 3-4 players and do the missions over and over again.
In Siren's call you might even be able to kill a few player heroes earning a reputation that can be used to buy +3 level SOs. A great way of saving infamy.
Once you're high level enough to enter Recluses Victory (lvl 40+) go there, get a heavy and start taking over neutral pillboxes with it. Get the heavy into melee range of the turrets to kill'em fast. The XP is just unbelievable, since you're actually killing lvl 54 bosses. If you win the zone, or pesky player heroes are taking pillboxes too, go kill groups of longbows and bosses with your heavy. It's still good xp compared to missions.

You should do this in teams though, since the über-reward for missions have a timer on it, so rotate between each others missions. Also, since the RV heavies are now heavily nerfed (can't be healed etc), the grind there is a bit harder.

RWZ lvl 35+ grinds
We now also have a new lvl 35+ grind zone, the Rikti War Zone. Borea gives out timed Rikti missions or hunt missions. Stay away from the hunt missions and grind the timed ones. If you solo them, just get to the end and finish the boss. If you're a MM or like grinds, kill all and make sure the communication officers spawn as many portals as possible since "defeating" portals give very nice reward. If you lack the skill, portals will get you killed by the way!

Note: The Pvp missions will be xp-nerfed in I8.