There are a few things you can do to keep a tournament going strong. Here's some background and a couple of suggestions...
I ran a weekly 10-ball tournament for about a year and the same guy won it well over half the time. It was at a bar with only one table so we did double elimination race to one. The bar has a strong field of regulars so the tournament never died out, but people did start to complain a little after a while. (Yeah, pool players complaining, can you believe it?) Anyhow, I didn't want to ban anyone from my tournament for playing too well, so I reworked the whole format.
I changed it over to blind draw scotch doubles 8-ball about a year ago and everyone has been loving it ever since. My thinking was that if the same one or two players still keep winning every week, then at least they'll be bringing someone random along for the ride! It's been way more social and the tournament can handle twice the amount of players since it's doubles. This is important for a single table bar since attendance tends to taper off if the tournament runs late, much like pouring too much water into a small glass.
The way I do the match-ups is to divide the players into two groups based on their past performance. Everyone from the top half gets paired randomly with someone from the bottom half. This keeps the powerhouses apart and gives the lower players a better than average partner most of the time.
Lastly, we also have a progressive 9-ball break pot that keeps players coming back week after week. It's a variation of the "must pot one on the break then run till you miss" break pot that I've seen here on az. I modified it so that you get 10% per ball made only if you've been at the tournament both of the previous two weeks. If you were there one out of the last two weeks then you get 5% per ball and if you're new or haven't been around in a while, then it's 2.5% per ball. This keeps things fair because the people who keep showing up and paying into the pot will get a bigger payout when they win, but it doesn't exclude new players from participating. It also has the benefit of keeping players coming back every week so they don't "lose their 10%" standing. Skipping one week could literally cost you $300 in the next week or two if the pot grows big and you go on a good run.
-Larry