If I see someone I know, I usually shout, "Hey! (Person)!" and then they look, and fall, and then I laugh.
If I'm alone, I just jam to whatever's going on in my IPod.
If I'm riding up with someone random, I let them start the conversation, I've met some cool people on the lifts.
If I'm riding up with buddies, we usually make stupid inside jokes, and shout "YARD SALE!" in barbershop quartet harmony when someone falls.