I had this website (www.Udustats.com)in my bookmarks and had forgotten about it until scrolling through them earlier today. A friend of mine from MilePoint (thanks Pat!) shared this link with me so that I could see what success people were having when it came to their upgrades clearing on United Flights. The database doesnt have an extensive history but there is enough data to show you what the trends have been.
A nice feature of the site is that it sorts upgrade clearance success based on your Elite Status with United so you can see how well each elite level has done in terms of upgrade clearing.
The site is easy to use and does not require any sign up or membership (you can post anonymously). I share this because I would like to see more United flyers add their data so that we can have more accurate and up to date information.
This site will help you see what the upgrade history has been on certain flights which in turn may help you build an itinerary that includes flights with better odds of upgrade sucess.
Let me know if you have any questions about how to use the site.