Automatic bidding?

I have noticed that on many auctions some folks have some sort of automatic bidding working, where if someone bids, their bid automatically increases, I assume up to a certain amount. No problem with that, but I must be missing something because I can't find how one goes about that process. How can one set up an automatic bid that increases with other bids?

Thanks!

For example - bid $1,000,000 - your bid will increase by the minimum needed to keep you in first place - unless the reserve was $999,990