Air flow in, air flow out, consistently sized wood & consistently WET wood (if you go for the wet wood smoking, which I rarely do but should start doing a bit more of).
Clarification: air flow means no obstructions on the vents & making sure they are all the way open. I have never really had any need to close them off unless I was trying to kill off the fire or close down the stack to prevent rain from coming in (i.e. shutting down @ night & the unit is still too hot to put the cover back on).
I don't have a bullet; I have a New Braunfels (mfg before they were purchased by Weber or CharBroil & moved the industry to China). Mine has the square firebox / grilling box on the R side & rectangular smoke box on the L side.
I also have a New Braunfels smoker (before they were bought). I have found that my temperature can fluctuate greatly if the fuel is too small, or too many smaller branches. I get most consistent results by putting a couple of large logs on the fire and leaving the vents about 1/2 way open.
Well, I was fighting the smoker for a while, but my dinner turned out really well. So it was worth it.
I ended up closing the smokestack almost all the way and the same with the firebox vent to choke the oxygen down, and my temps leveled off and did much better.
I have a BBQ Guru that I use with my Big Green Egg. The guru is actuall a fan that fits on the bottom slide vent of my egg (can by pieces that conform to most smokers). It has a thermometer probe that can clip on to the grid. If the temperature starts to drop the fan comes on, bringing the temperature up. I find this helpful when smoking large briskets overnight. Just plug it in, throw the brisket on, and never have to check the temp.
a few things --- a baffle helps distribute the heat
if yours is upright, it's easier to regulate temp, but just get to know your hot/cold spots. try cooking a package of biscuits to test it out.
a waterpan helps regulate the temperature. if you want a hotter chamber, fill your waterpan with sand. it'll accomplish the same purpose (but it won't collect the drippings), but the sand will keep a higher constant temperature.
don't open your door too much -- even if it is fun to peek and see how the food looks.
buy a remote thermometer. (the ones they come with suck) you can drop it through the smoke stack, and it'll keep more accurate readings...then you don't have to open the door to check things stuck in the meat.
I wouldn't use the smokestack to regulate your temp. The smokestack should remain fully open at all times. Decreasing the smoke circulation could lead to creosote on your meat.
What kind of temp gauge are you using? Plenty of stock temp gauges suck ***.
The equipment is the secret. The best pits are welded and, therefore, sealed. The nut/bolt pits work great, but there are plenty of seams for air to affect your temp. I've heard of guys using roofing sealant on their NB smokers to seal them up. Colder and windier days are a little harder on temp regulation as well.
Keep in mind the outside air temp & sun exposure as well. Since I live in Colorado, it's very very difficult for me to smoke anything this time of year and if I try I have to use a LOT more fuel. In the summer and when my unit's in the sun, the sunlight alone can sometimes heat it to well over 100 degrees.
Pretty much all about air flow for me. That and size of the sticks. If I want a long term constant temp, I make sure that I have a good bed of coals, drop a nice thick (dense wood is really good for this too) piece in the box, and then cut my air flow back (and then go in to the game). It'll slow burn with a consistent temp for hours.
Not sure if this is a problem make sure all old coals and ashes are removed before you get started. THis will help with airflow. Make sure you have a good hot coalbed before you start cooking. This will help temp stay steady. Keep doors closed.