What juice are you using? If it is 100% PG then you may get little vapor when you exhale. PG vapor dissipates quickly.
I like juices with at least 30% VG like 70pg/30vg. PG lets through the TH of the nicotine, VG smooths the inhale and adds more visible vapor that hangs around a bit longer. Try 50/50 for even more vapor and less TH.
I am getting a lot of vapor with a 510 + cartomizer and 50/50 juice (a juice I have that only was offered in 50/50).
Cartomizer I am using:
http://www.litecigusa.net/510_Blank_Cartomizers_p/510bogesoftcartosb.htm
To see the most vapor on exhale, exhale right after you inhale. The longer you hold it in your lungs, the less vapor on exhale.
And know how to use your batteries to get the most vapor.
Drag slow - dragging fast just dilutes the vapor with air
Manuals (preferred) - press in, about 1/2 to 1 second later start dragging.... before your drgais done let up the button and drag for another second to get out the rest of the vapor
Auto - if the atty is cold do a fake short drag first to warm it up, then your real drag
Why do you want more vapor on exhale?