There are a number of things that you can do to help when requesting information by letter, phone, or e-mail that may improve your chances of getting a positive response:
Be as specific as you can on what you want, without being wordy
Don't give unnecessary details, unless requested by the other individual(s)
Provide an enclosed stamped, addressed return envelope, if the request is made by mail
Include a few dollars (cash works best) to cover reproduction costs or as a small gift for their time, if the request is made by mail
Offer to send money to cover the costs of mailing and photocopying if the request is made by e-mail
Offer to share your own research if you are seeking information from another researcher working on the same family line
If the request is being made to a courthouse or library, ''walk lightly'' as these folks have other duties and may be ''hounded'' by genealogy requests. Again, do not overwhelm them with family trivia. They've heard it all, and are probably not interested.
Give them your e-mail and/or phone number (and tell them to call collect) if they have questions concerning your request
If there is truly ''research'' involved, perhaps you should inquire if there is someone who has time to do the research for a modest hourly rate. This may turn someone to the task, or it may entice someone to take on the task for you for no cost, because it is obviously important to you.
Remember that it would likely be expensive for you to drive/fly to the location, stay in a motel, to do the research yourself. As such, you can afford to incur some costs to get the data ''remotely.'' If the information isn't worth the money to you, then you shouldn't be requesting someone to get it for you. This seems to be a good litmus test.
I used to provide postage and photocopying for free when responding to researchers' requests, but it has gotten to the point where I frequently receive requests for information. If the requestors don't offer to pay for the costs, I now tactfully ask if they would send me money to cover the postage and reproduction costs, after they receive the requested information. I think folks don't realize the genealogists can get a lot of requests and collectively, incur a fair amount of costs in responding over a period of a year.
My success? I have had only one case in which I enclosed cash to a researcher, asking for specific information that I knew she had, and got no response. It cost me five dollars. On the other end, I believe I have just encountered ''being stiffed'' for the first time by someone who said they'd pay me for postage and reproduction costs upon receipt of the information. Perhaps this event gave me the incentive for this article. Again, I'm out only about five dollars (postage costs is self-evident on the envelope and I figure any good researcher knows about the average costs for photocopying, so I don't present a bill with the material).
I think most genealogists are honest, particularly if you put your trust in them ''up-front.'' Most folks find it hard to betray a trust and most genealogists share common bonds of interests.
In response to some e-mail exchanges, I once sent a small check to an individual who said that he'd research a couple of county courthouses for me that were several hundred miles away from me. He was local, knew the clerks, knew their files, and could do it cheaper, faster, and better than I could. The check was intended to be a deposit to cover initial research costs. Instead, he visited three county courthouses, found what I needed, made photocopies of the relevant materials and mailed them to me. When I went to ''settle-up'' with him, he said that the deposit was sufficient. There is little doubt, that his hourly wage for the work was a small fraction of minimum wage. He worked cheap, enjoyed the work, and probably was happy to earn a little money from the skills he learned from his hobby. He was now a professional. He is also an honest person.
I can't imagine the amount of requests that courthouses and public libraries get in the eastern US from folks who are trying to track down ancestors. Probably most researchers don't stop and think of the imposition of the request. If theirs were the only one, it wouldn't be a problem, but when bunches come in, it can be pretty demanding.
Of course, I cannot make any warranties that your efforts will be more successful, but I can attest that I believe my success has been better, after adopting these few ''rules of the road.'' Plus, from evolving from primarily being a requestor of information, to being one of primarily responding to information requests, my perceptions have changed a little.