2. What materials should I give my letter writers to help them write a well-informed recommendation?

The materials you give your letter writers should help them write letters that are personalized as well as customized to the field and programs you have chosen. Include a copy of your statement of purpose (give them a rough draft if that's all you have), a copy of relevant class and employer evaluations, papers you have written for them (with their comments), your resume, and any other product or description of relevant accomplishments. Even provide a brief outline of what points you would like the letter to include.

You can request that letter writers address certain skills or achievements about which they have first-hand knowledge and/or use letters of recommendation to compensate for a weaker part of your application. For instance, let's say you tend to do very poorly on standardized tests so you've received a low verbal score on the GRE's. You have an instructor, however, who has consistently praised your verbal abilities. Make sure you specifically request that s/he address this in his/her letter. Yes, s/he can even acknowledge your trouble with standardized tests. It is wise to put any request like this in writing so the letter writer won't forget. Note that you can't benefit from this if you didn't take the GRE's early enough to have the results already!