My technique's pretty simple. Fight enemies you have no business fighting. It can be difficult, having to heal after every combat or every other combat, but the XP gained is higher. Also, keep turn length as short as possible. Shorter turns = more turns = more XP.
Other factors are important, too. Always know the strengths and weaknesses of the chosen enemies, and base your attacks on this knowledge. (The wiki is decidedly important to this.) Always choose the best gear possible (again, making certain to keep turn lengths minimized) and get it ASAP. The AH can be useful in this.
The importance of choosing the right class is also key... gadgeteers are quite good, since high ref makes ranged weapons work well (but at low levels, ranged isn't much use unless you're using the bracers as a talisman). Also, when playing a gadget, the choice of buffs is important. Always go for improved grip on the weapon, and improved reflexes on the armor... this seems to be the most effective combination. Psions are the best at really low levels (aura:vim makes them the most effective fighters early on) and at higher levels, their spellcasting is impressive, and they get healing early. Note for psions: try to get the Thunder in a Bottle at the earliest opportunity, as psion spells are insanely expensive in PP. Elems aren't as effective at the lowest levels, but once they have lifeblood and firestorm, they are deadly. Natties... are good for a challenge. They're less useful at low levels and have no healing skills, and while they do have many of the best buffs, they really aren't a good choice. If you insist on playing one, the puppet is almost mandatory equipment, as fumbling can be fatal.
Also key: make friends in chat. Buffs from other classes are practically mandatory for efficient leveling. Also, the right people will make things available to you that you wouldn't otherwise have access to. For example: liao is without a doubt unmatched for levelling. The only way any lower-level character is gonna get it, though, is buying it from someone of higher level (or being given it) so it's useful to know people who might be willing to sell/give it.
White pills can also be used to good effect. Yes, the longer-lasting wretchedness can be problematic, but use the extended wretchedness turns to farm HP. This will make surviving against tougher enemies more likely... and the euphoria phase is identical to liao.
For best results, from level 29 onwards, spend time in Insanity while euphoric, or (if you have access to TNT) fight in S7G. Also, the depths of space are a good place to gain XP, if you can survive.
As soon as you reach level 40, get and equip the trident. This works best for gadgets (no fumbles is exceptionally important) but it's a good weapon for anyone.
That's pretty much everything I have to contribute on quick leveling strategies. I'm sure others have even more (and some might even contradict some of what's here) but this is just based on my experiences.
One last thing: I mostly stayed away from suggestions involving IotMs (other than, of course, finding someone willing to recyclonize for you to get TNT) but obviously the more old IotMs you can obtain, the better off you'll be. They really do give a hell of an advantage.