Years ago I was listening to a podcast from Michael Ardnt, writer of Little Miss Sunshine, and he was talking about the process he went through to get the script into shape before giving it to a producer. One of the tools he used was a questionnaire. His reasoning being that studios spend money on test screenings to find out what works in a movie, why not do the same for a script. Interestingly the questionnaire allowed him to get intelligent and useful feedback from people who weren't necessarily familiar with screenplays. He divided the questionnaire into 3 parts.
For the first part he listed the top five or so main characters along with a brief description and asked for a 1 to 10 ranking (1 being unfavorable and 10 being the most favorable) along with general comments.
He then did the same for the plot/story, listing the top seven to ten plot points in the film -- describing each in brief and providing page numbers for reference. He applied the same 1 to 10 scale and left room for comments.
The last section of the questionnaire focused on what he had self identified as problematic within the script. Again, here he lists out the top ten or so script problems that he's noticed and asks for rankings. Here though, 1 equals not a problem and 10 means this is a severe problem. This last section is very important because in his (and my) experience when you admit that there's a problem in the script your readers are less concerned about hurting your feelings and more honest about their reaction to the material.
The other great thing about doing a questionnaire is that you can get scientific about what's working and what's not working. You want your main characters to all be in the 9 to 10 range. You want your plot points in the same range as well -- and if they're not, you know exactly what you need to work on.
I like to use this tool after I've done a few drafts and have started to run out of ideas. By pooling a large sample (7 to 10 readers) I've got a better handle on what I need to fix and in the best cases, some possible new solutions via the comments on the questionnaire. If you're stuck for readers, try Facebook. Seriously. I used a similar social networking site last time to recruit readers and thanks to the questionnaire was able to solicit quality feedback.