Time. That is the accessory I find hardest to come by.
A digital thermometer to read the pit temperature at the grate. Most of the dial thermometers are inaccurate (except for Tel-Tru). The thermometer doesn't give you the temperature where the meat is.
So much this, with a side-addition of suitable weather (not too hot, cold, raining, etc).
Thermapen - worth its weight in gold for checking temps quickly
Good, sharp knives
Full / half pan aluminum trays with foil - super handy
Patiience - in LARGE doses........you'll need this to pull it off.