oh boy... this could be long, so grab a seat.
well, there are just so many things; let me try and group them by the senses:
Sight: Erotic Artworks (like manga and of course hentai) portraying beautiful women regardless of age (Like Spalose, I don't believe girls are too young to be objects of fantasy), the sight of their grace in motion, the right posture, the light effect on their long, silky hair (ahem... losing track, sorry), in short, arousing things I can perceive, this of course includes such elements as Lingerie, a sexy smile, a sexy walk (there will be some who don't believe such a thing as a sexy walk but there *is* such a thing), a woman undressing, a woman only wearing my shirt, etc.
Hearing: the quiet sound of a woman being sexually pleased (I *strongly dislike* ordinary Porn's stupid howling and cursing, and I find it hard to withstand Japanese porn's ceaseless yelping), the dialog between lovers, the recount of sexual experiences, too.
Scent: a woman's scent, whether along with her perfume or natural.
Taste: any piece of food that an attractive woman feeds me (specially if she uses her mouth to do it), and of course the taste of her skin (however, there's a point in which the taste of sweat becomes unpleasant)
Touch: caresses to the palm of my hand; the feeling of a woman's hand lightly and sexily scratching my back, Passionate, deep kisses, and of course, fellatio.
Mind: remembrances of experiences, Fantasies of the things that aren't possible but still are arousing, fantasies of what might happen