I've found the fastest way is usually to find the turn in that is easiest produced, which frequently means something that requires few mats, preferably easily acquired mats. Right now I'm leveling CRP which has a triple turn in that you can do in Gridania with the turn in also in Gridania, and all you have to do is turn in 5 Walnut Lumbers. And with BTN already leveled to 41, I can farm a stack of Walnut Logs in no time and churn out Walnut Lumber like nothing. I'm burning from 25-30 in record time just doing this one Leve.

Also, if you have BTN leveled you can easily farm Cotton and Flax, which make leveling WVR very easy. So far the only thing that slows me down on WVR is when I need to farm Diremite Webs.