I can level my characters somewhat fast, the general rules I've always followed are:

Quest in an area just a level or two under you.


If there are multiple areas for you to level in at your level, do the easy ones in one location first, then head over to the other area and do the same.

Spam your LFG for any Elite quests while working on another quest, so you're not just standing around.

Only run an instance 1-2 times for items, and only if you have quests that can be completed there (unless an item is a SIGNIFICANT improvement).

Avoid "delivery boy" quests that drag on forever, as they're more time-sinks than anything else.

When in a rough spot as for quests that you can complete, grind it out until you can move to another zone or can complete them.