Asking someone to donate their money for the cause you are serving is a difficult task, but it can be well done by approaching people in a right away.
Generally, you do not approach people for seeking donation for a charitable cause in person. A letter is the most commonly accepted method, whether you post it or email. So write a letter in a manner that convince the potential donor why he should contribute towards the cause you are serving.
Start the letter with an introduction about your organisation, the cause you are serving, and how long you have been in the social field.
Explain in the second paragraph how important is to work for this cause, and what could be implications if this particular cause is left unattended because of the lack of funding from public sector or otherwise.
Also, mention how important is the contribution from the potential donor you are approaching and that there are many other people who make donations to your work and organisation. Further, if the person has been contributing to your cause regularly every year, give the figures or numbers of clients that you have served with the money.
Highlight your donation collection program, and its pros and cons, and also invite the potential person or head of the organisation to personally see your work and meet your client.
Close the letter with salutation, your complete address, and do not forget to sign it if you are posting it. In case of email you should have business signature in your email account so that the potential donor immediately recognises you and your organization.