I'm curious as to why you think that the relevant factor in your upgrade success is codeshare flights. You state that every time you fly LAX-FLL it's a codeshare and you are low on the upgrade list. So are the flights that you are taking where you do get upgraded the same route? the same time? the same day of the week? etc. etc.

There are many factors that affect upgrade percentage on flights, some of which might be related to codeshare or not, but to me it doesn't seem to be the case.

Despite my skepticism about your reasoning, I do have a pretty good answer for you.

The best to avoid codeshare flights is to use any of the flight schedule resources from SkyTeam or any flight schedule resource that will show you multiple airlines. You can go to skyteam.com and find the Find Flights link to do a search online. I have an application on my Treo 755 that is a Palm OS app for the SkyTeam timetable. These timetables will generally show all SkyTeam flights between the cities you choose, when you see flights from multiple airlines for the same times, you know you are looking at a codeshare.