1. The very first scene of the movie, with Martha Vickers, sums up what the movie's about. There used to be a rumor that Vickers' part was cut down because Lauren Bacall was jealous of how good she was, but I don't think that's accurate -- what actually did happen is that new scenes were shot with Bacall to give her more fun things to do in the picture. Vickers herself obviously should have had a much better career. You can't always believe what Hawks said, but he claimed, plausibly enough, that he urged her to continue playing bad-girl roles, but instead she went back to playing ingenues, in which roles she just seemed like any other pretty Hollywood actress -- she couldn't do the unique things she did in The Big Sleep.
2. In what is quite rightly considered one of the sexiest scenes in '40s movies, Humphrey Bogart goes into the ACME book shop (yes, Warner Brothers used "ACME" for everything) and meets bespectacled clerk Dorothy Malone. ("I'm a private dick on a case.") Originally Malone was just supposed to deliver some exposition and be done with it; Hawks said he expanded the scene because Malone "was so damned good-looking."
3. Bogart gets into a cab to do (as I said earlier) a very standard follow-that-car scene, except his driver is a woman, Joy Barlow, who is clearly interested in more than just tailing a car.
4. Bacall -- who's really kind of overshadowed by nearly everyone else in the film -- sings "And Her Tears Flowed Like Wine" while both she and Bogart take appreciative glances at a randomly-appearing (and, once again, clearly interested in Marlowe) attractive woman.
And I haven't even gotten to Mona Mars, or Agnes, or other Big Sleep women (SleepFemmes?).
So many women throw themselves at Bogart throughout the movie that you almost get the sense that Hawks is making some kind of meta-joke about Bogart's unlikely sex-symbol status; he'd only just recently established himself as a romantic leading man (in Casablanca and Hawks's own To Have and Have Not), and it's almost as if Hawks is saying: well, if we're expected to believe that Humphrey Bogart is a sex symbol, we might as well take it to an extreme.