A side note to the "25% is industry standard" comment.
This is incorrect only because there is not a "standard" percentage: it's all dependent on how difficult that sales agent thinks your film will be to sell.
You can get a very good domestic sales agent (AKA producer's rep) for between 5-30%
Good foreign sales agents are anywhere from 15-40%
I have just recently (last week!) secured two sales agents, one for each of my indie films that were both shot on DV and have no recognizable names but have decent festival attendance (just not the big ones).
For my horror feature I was able to get a low percentage because it is good enough that it will be an easy sell.
For my indie drama the situation was much worse, even with decent festival attendance. It is a very difficult film to market and sell, and therefore I had much less room to bargain. While we did settle in the middle of these percentages, it was still an eye-opener to see how different the situation was for these two films.
My advice is to see what numbers keep getting thrown at you, and then try to get repped for at the very least 5% less than the average. The worst that can happen is that they can say no. The beauty of this business is that a sales agent will not cool to your film most of the time. Product is product, and if they think they can sell your film today, they'll still want it next month. For these guys, it's all about filling up their slate so that they have enough product to bring to the markets...
Good luck,
-Jim