Try the Minion Method. Don't just start with a big ol fire. Also, if your temps rise, slowly start to close off the bottom vents (always leave the top open) but just close and open as necessary a little bit. Then wait 15 minutes and then alter again if necessary. Here's something I copied off of another site that explains the Minion Method pretty well...
Sometime back in 1999 or thereabouts Jim Minion was participating in a regional barbeque championship in the Pacific Northwest. His cooker of choice was a Weber Smokey Mountain.
However, following the manufacturer’s instructions on building a fire in this otherwise wonderful smoker proved useless as the fire would quickly shoot up to well over 325 degrees. What to do, what to do?
Jim Minion, a fleet manager for an auto sales company, tried something different – he spread a layer of lighted briquettes over a pile of unlit briquettes and he found that he could maintain a steady fire for as long as 22 hours in his Weber Smokey Mountain. He took a first and a second in two categories that day and the Minion Method was born.
About that same time I was having incredible difficulty holding a steady temp for any decent length of time in my Hondo offset. I came across a description of the Minion Method on the Internet and decided to give it a try. I filled the firebox with Kingsford briquettes as recommended, lit a Weber chimney filled with briquettes, dumped them on top and for the very first time I held a rock steady 220 for four hours, but then the fire choked itself out from all the ash produced by the briquettes. But heck, that was a whole lot better than before.
My wife’s uncle, one of the most fun individuals I have every had the pleasure of knowing (he was one of those people who, from the moment they walk into the room you know you are about to have a great time), and a true lover of ‘que was visiting and he wanted me to fire up the barbeque. As an incentive he brought me a bag of lump charcoal. Not wanting to insult a guest, I fired up my Hondo using the Minion Method with the ump charcoal.
I fully expected a disaster as everything I read about the Minion Method said to use briquettes. Instead I was stunned – I quickly got the fire settled down to 220 and it stayed there – and held – and held – and 8 hours later the temp was still reading 220! By then I was done and removed the meat from the smoker
but it was another two hours before the temp dropped.
A convert was born!
That was several years ago and I’ve learned a lot since then. Most important is that not all lump charcoals are the same. Some will only hold a steady fire for about 4 hours. The average lump will give you about 6 hours. The best lumps will hold 220 for 10 hours or more. Other things that will affect the burn time are outdoor weather conditions, the make/model of smoker you have, and the temp at which you are cooking. I have also learned that once you learn the individual quirks of your smoker you can “dial in” any temp you want by making small adjustments to the chimney damper and/or the air intake control.
For the longest, steadiest burn times I recommend you get the best quality hardwood lump charcoal you can find. Briquettes will work, however they produce so much ash that the fire chokes itself out within about 4 hours. High quality hardwood lump charcoal burns hotter produces very little ash resulting in a much longer burn time.
If you don’t use a charcoal basket, you need to find a way to keep the charcoal away from the air intake. Here is a good way;
Fill the firebox with charcoal all the way to the lip of the opening between the firebox and the cooking chamber then hollow out ever so slightly — about an inch or so — just enough to make the pile slightly concave — a small area in the middle by pushing the charcoal up around the sides a little.
Fill a Weber chimney with charcoal and light it. When it is going real good (all coals glowing) then pour it all on top of the charcoal in the firebox, keeping it centered as much as possible.
Close the lids but leave all the vents (air intake and chimney) wide open. When the temp reaches 275 – 300 degrees, begin closing the air intake. Close the air intake half way then check the temp in 15 minutes. If it is too high, close the intake half way again and check in 15 minutes. If still too high, close the air intake all the way. Check again in 15 minutes. If the temp is still too high and ALL VISIBLE SMOKE DISAPPEARS, begin closing the chimney — you guessed it – half way. Check again in 15 minutes, etc. At some point the temp will stabilize — check the vents and remember where they were as that will be your starting point next time — in other words, after the initial temp has reached 275-300, then you can close the vents down to your starting point rather than repeating the entire procedure again. The fire will slowly burn down through the pile of charcoal providing a nice, long, steady burn.
So, all of us backyard pitmasters owe Jim Minion a huge thumbs up for daring to try something different and making top notch barbeque a breeze.