Okay, clearly this is an important topic, and also a tough one because there's so much room for variation. I'm pretty vaporized so this post is under construction.
First things first, make sure you have everything specific to your unit set up correctly. Refer to the instructions or ask around for tips on that.
Grind or break up your herb
Exposing more surface area for the hot air to pass over = more to vaporize
Load the herb properly
Another one that varies between models. In general, it's probably good to have at least enough herb to completely cover the screen. This is because the hot air takes the easiest path; if there's a spot with no herb, most of the air will go through it and not your herb. Don't pack the herb too tight, keep it fairly loose so that the hot air can travel through and pass over more surface area.
Find the right temperature
This can be tricky initially because it is affected by your draw speed (need to expand on this later)
Finding the right draw (inhalation) speed
(need to expand later)
Stir between hits
(need to expand later)
More tips
I've noticed that often times, the first draw or two do not produce very much visible vapor. This doesn't necessarily mean that you aren't getting anything, the light vapor is probably the cleanest.
Also, I think that sometimes the herb may need to dry out a bit first (during the first hit or two).
So don't get discouraged if you don't get a huge cloud with your first hit off a fresh bowl. Stir it up and keep going at the same temp. Usually my thickest vapor clouds are on the 4th+ hit.
Other things that can affect vapor consistency
Quality of herb
Will add to this post later.