How to reduce the load of professors writing recommendation letters for graduate school application? If a student can't decide which school to apply at one time, and different schools have different' deadlines, how could the student do to reduce the load of professors writing recommendation letters for graduate school application?

One possible solution. I heard of interfolio (http://www.interfolio.com/). Does anyone know if it might help? My concerns are that each school has its own only application system, and it might not be good for recommendation letter to not have specific school name in it. What do you think?
You can reduce the amount of work the professor needs to do by following this excellent advice from Prof. David E. Keyes.  Here are some of the most relevant bits:

To ensure that such a writer is well primed to execute your reference efficiently, you should create a self-contained packet containing all the information the author will need to dispatch the reference in one sitting: (1) contact information for the recipient of the letter, (2) a description of the position and application closing date, (3) your own application essays and cover letter, (4) a resume, (5) relevant transcripts and scores, and (6) an explanation of the niche of the writer! It is very useful, as a reference writer, to receive a reminder along the following lines: &quot;Professor Keyes, your letter will be the one that comments most authoritatively about my analytical ability, my promptness in completing projects, and my reasons for wanting to pursue X next fall. Remember that you gave me an A– in partial differential equations two years ago and it was your suggestion that led to my summer at Los Alamos with Y.&quot; You should provide this packet in both hard and soft copy. Writers of lots of references maintain files that may be hard or soft, or both, and you should make it easy for those writers to locate your files quickly for subsequent updating and future requests.
Some faculty write many dozens of letters of reference during peak months, and they may even ask candidates for sample text to be incorporated into letters, to ensure that they capture their niches. You should not be flustered at such a request, and should not be modest in complying. You should be aware, though, that your words will not pass directly into the delivered product; they will be used simply to get the author's juices flowing following the formulaic paragraphs of the letter.
To help maximize the time a writer has available for each candidate, you should complete the reference process as far as legally possible yourself. For hardcopy applications, this means printing out and filling in as many fields as possible, both about you and about the writer (title, affiliation, contact data). For reference letters collected online (the vast majority for positions filled by junior candidates), you can usually fill in each writer's basic contact information. Such streamlining is always appreciated and leads to fewer errors and faster completion.

I don't think something like Interfolio would help, because the last thing you should do is force the professor to learn to use a new tool just so he/she can write a letter for you.