I was taught stick it on whenever the tachometer is out of the green arc (on the C152 that's about 1900 rpm), push it closed about 100ft above the tarmac and on the ground have it open when doing the idle power check, keeping it open until I want to move off (yes carb icing can form on the ground, and half way down the runway is not a good time to find out your not getting full power!)
My biggest problem is remembering to do this since I've been flying the fuel injected C172R - the carb heat is replaced with a panel light dimmer knob!!!