what do you do when your boss makes a pass at you?

I slapped mine in the face, because I was a minor at the time!

If it happens again, you need to tell him/her no, sorry not interested, you're making me uncomfortable etc.(Assuming you're in the US, and also assuming it's unwelcomed). Then, from that point on(including when you told him/her)you must document everything that happens, dates/times, any retaliation that's taken. You have to have that if you end up having to file a discrimination/harassment complaint. You have to afford them the opportunity to stop by telling them so. If it continues, go above him if possible, otherwise contact the feds and file a complaint with them.