While I think Baldric's estimation of leveling time is a bit fast, I can tell you that it's pretty achievable to hit the "level 35 in 35 hours or less" model, and that's where you really begin to have to utilize your specific class / build's strengths. But beyond that, I've found that two things can dramatically speed up the levelling process.
The first is working with other people. The experience bonus (and decreased downtime) associated with working in a group is pretty substantial. For example, two melee characters can trade turns starting combat when the other is dirt-kicked (which happens a lot in the middle levels, until you learn to aim low and can quickly mangle limbs). Two magic-driven characters can take turns "tanking" or providing a tank, and a melee / magic class is really what it's all about, although healers need to get used to seeing their tanks drop into the 40s and 50s on basic NPCs, and save their own mana for mana-efficient attacks.
The second is understanding the more efficient ways to level.
Toward caster classes: magic missile, cause light, lightning bolt, and spear of faith are the primary leveling tools on single opponents, and flamestrike / call lightning are sick with groups; but don't underestimate how much group experience you can get just from frenzy and a dagger when you're out with a buddy. When another PC starts combat in an area with non-aggressive NPCs, you can (and should) safely stand beside and time spells between rounds as much as possible.
Toward melee classes: strong, slow weapons are now the best choices for efficient leveling. (No-longer-recent) changes to PE mean that a fast but weaker weapon drains you more quickly. Also, enchant weapon is the most important thing you can have someone do; and unlike the reviled enchant armor Skinner Box, enchanting a weapon just to humming is something a caster can do in the span of two ticks easily, and will save you 5+ real hours in game.
As an example (stats taken today):
Code:
oOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOo
o [Scores] o
o===-===-===-===-===-===-===-===-===-===-===-===-===-===-===-===-===-===-===o
o Status: Champion(1) Race: Giant Class: Barbarian o
o Kingdom: Wastes Cabal: Independent o
o===-===-===-===-===-===-===-===-===-===-===-===-===-===-===-===-===-===-===o
o Coins: 17 obsidian, 4 platinum, 9 gold, 5 silver, 4 copper o
o Carrying: 1/17 items Load: 255.4/569.0 pounds o
o===-===-===-===-===-===-===-===-===-===-===-===-===-===-===-===-===-===-===o
o Health: Excellent Mental: Lively Physical: Worked o
o Attribute points: 8 Wimpy: 15 percent Mode: Kill/aggressive o
o===-===-===-===-===-===-===-===-===-===-===-===-===-===-===-===-===-===-===o
o You are resting. o
o You have just begun your journey to the next level. o
o You have played 54 hour(s) so far. o
oOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOoOOOo
Not my fastest (Celeria, an elven warlock rolled and power-leveled during an SK bonanza with major support from a Miran priest, hit GM in 14 hours), but given that this character has repulsive charisma -- if he can do it, so can you.