Call Of Cthulhu comes with a DM screen that summarizes the most important rules, at least the edition I had.
As an idea, the things I had to look up most frequently (and under time pressure) were rules on sanity loss, injuries and healing, combat maneuvers, the list of weapons.
I also like to have a long list of NPC names I can just draw from whenever the players decide that a random passerby must be plot-relevant. This can be combined with a list of quirks (limps, shy, drunk, religious, frightened, foreigner, gorgeous,...).
Pick a name, pick 2 quirks, BAM, instant NPC out of the box.