Follow the paparazzi. They will know where the biggest stars are and will be waiting outside for them. Get out your camera and you can blend right in.
Hang out on Main Street. Celebrities are going from parties to interviews to swag events up and down the street all day long. The best way to meet celebrities at Sundance is to simply walk along Main Street. The most popular stops on Main for stars include Fred Segal and Delta's Sky360 Lounge.
Hit the slopes. Celebrities eventually tire of parties and premieres and take advantage of the excellent Utah skiing. Popular spots are the Sundance Resort as well as ski lodges in Park City.
Attend panel discussions, where celebrities are often asked to speak. Even if you don't sit in on the discussion, you can go to the venue and wait outside. Usually, celebrities will be willing to take pictures with or sign autographs for fans as they leave the event.
Attend movie premieres. Often premieres are sold out, but even if you don't have a ticket, go anyway. Sometimes stand-by tickets are available. Most film premieres will have a red carpet entrance for the stars. Usually, fans can wait near the carpet and have the chance to meet celebrities as they come in.