Agressively. Base them on your experience from watching other auctions or on gut feeling. If it doesn't sell at the price point you want, you can always drop the price (although this is not recommended since you may get a reputation for doing this) or hang onto it for a few more months, hoping to try again later.
If you just want to have fun and watch the fur fly, price your opening bids low -- items will climb up to a natural point in one or two weeks. The only danger in a low opening bid is if there is only one bidder. Ask yourself, If only one person is interested in this, will I be happy selling it at this price?
If you have no idea how to choose an opening bid, contact an expert in the area or consult a comprehensive set listing for hints.
8 Jan 96 [TSL]