Like RonL said "patience" I don't worry over a temp drop or spike, unless it gets 25 degrees out of where I want it. If it gets out of this range i will open or close the intake just a little bit and wait at least 15 minutes for it to settle in. If your temps are falling after 5 or six hours and you still have plenty of fuel... give the smoker leg a little kick or a whack with a stick to knock some of the ash off of the charcoal. sometimes this helps.
18.5 WSM, Green Weber Genesis320, Cast iron firepit(w/grill), and a Copper Weber 22" OTS