Doorknock early, doorknock late.  Carry a water bottle with you.  I found one at WMT that let me clip it to one of my belt loops.  It might look a little odd, but it kept me going. 

Your best times to doorknock are first thing in the morning 8-10 and then in the afternoon from say 4-7.  Doorknock businesses during the hottest part of the day.  Most of them are air conditioned.  If you're focusing specifically on houses, look for shorter streets when it's really hot.  That way you finish quicker and get back to the car and the A/C.    You also have to recognize your point of diminishing returns.  You look stupid walking around with pit stained clothes sweating up a storm.  It's OK to sweat a little, but when you look like you've been running a marathon, it's time to call it quits.