1) Hard to explain, but here goes. When the guy and girl (or two girls) are kissing either clothed or half-naked, and you see the girl sort of shifting and trying to rub up against her partner, especially popping a leg over her partner's thigh to grind on while they kiss. It's "animal in heat" instincts kicking in, and I find it hot as hell.
2) If the cock first appears floppy or only half-hard, I like seeing it become hard because of what the girl is doing. I quite like seeing a girl envelop a cock in her mouth, and when it slides out again, it's longer and fatter.
3) Those shots where a girl is sucking a cock and you can see her rubbing her pussy with one hand as she does.
4) Rimming. I love seeing the girl get rimmed because I like to do it, and to be fair the sub-conscious "you dirty whore" aspect of seeing a girl rim a guy seems to speak to my cock as well.
5) A visibly, proper wet pussy. Aroused wet, not spit-wet. Shiny wet before a cock or mouth goes near it. Or during penetration, when the cock is shiny and slippery looking because of how wet the pussy is that it's pounding.
6) Pop shots. I know they only really come at the end of a scene, but the initial sight of a cock going off on a face or some boobs will make me hard, and want to watch more porn. Blow Job Relief Fund is one of my favourite scenes ever, because there is a "flash forward" facial within 90 seconds of it starting, and then a length sixway sucking and fucking... which of course, ends with another pop shot.