Request letters are a good way of seeking help and you have to craft them well, so that the reader does not feel you are imposing or overbearing. Sometimes, you may find yourself in a delicate position and it is helpful to remember a few points before you start writing request letters.
Request letters are unsolicited letters, so that very fact should make you use a very polite and courteous tone. Start the letter with a statement of why you are writing the letter in the first place and then make the request. Be clear and straightforward about what you are seeking.
Ideally these letters should be short so that you are not impinging too much on the time of the reader. It is good to keep your requests within reasonable limits.
Though you must thank the reader for their time and effort, do not thank them in advance for what you expect from them. This can seem very presumptuous and may irk the reader.
Convey your sincerity and also offer to reciprocate in any which way you can.