I agree with Airfiols:
when you finish cooking, close all the vents to extinguish the fire immediately so you save as much fuel as possible so you can use it for your next session.
On my next cook I scoop up the used charcoal (yes it's messy), put it in my chimney and then top it off with new charcoal. LIGHTER FLUID IS EVIL!! Messy is better than bad tasting food! Don't use it!
I also have a WSM but do not reuse charcoal in that. I take to used charcoal from the smoker and use it in my kettle. By using used charcoal in my smoker I have had temp control issues.
By the way - I use kingsford charcoal.