In bullet and blitz there is no time for deep calculation; intuition and patterns come first. An opening system you know and play comfortably helps a lot.
With little time choose safe moves, and look for positions that cause your opponent problems but carry no risk for you. If your opponent is short of time, keep the position complicated.
Fast games are fun, but to improve, don't neglect playing a few long games (10 minutes or more) each week.