The route from Seljavellir is the most common one. It’s mostly a gentle slope all the way to the top, and supposed to be brilliant for skiing down (although I haven’t tried that myself).
Another option is to start from Skógar, go up Fimmvörðuháls, there you can check out the new mountain witch was formed in the eruption just before the Eyjafjallajökull eruption. From there go west to the Eyjafjallajökull. It is considderably longer route but on the plus side there is a nice hut on Fimmvörðuháls (the one Útivist owns) where you could spend a night.