I had a pretty spec UDS, and initially had trouble with maintaining 225. I made a few small changes, then one large one. My first change was to ditch the budget grill ash pan. I replaced it with a pizza pan - it cleaned up the air flow at the bottom of the pit. Second change was to make 100% sure their was no extra humidity in my lump. After that things went much better.
Then I said to hell with this all and put a BBQ guru on it. I think I have managed a 24hr burn with 1 basket of coals (I did burgers for lunch, chickens for supper, and then pulled pork over night. Most of the time I just walk around the yard picking up limbs that have fallen from my by pecan trees - I can get a 6 hr burn off a basket of broken twigs. The guru is true set and forget - not only I can control a burn of twigs with it, I can usually go from empty to a locked in temp in 15 minutes (abet that is a bit of trial and error - you have to get a good burn started w/o overheating the pit).