There was a discussion on the VaporTalk Vlog about the fact that you will vape more than you smoked, and the biggest recommendation they gave was to prevent over nic'ing yourself was to cut the nicotine amount down alot, they recommended to use 12mg to 14mg.
That seems to hold true with my experience. I started out with 12mg and was puffing near constantly, so I tried upping it to 24mg. Then, to my surprise, I was still puffing constantly and getting headaches.
18mg is next for me, and I'll see if I can't just reign in my novel new vaping habit a bit.