Your first step is to actually know exactly what you are asking for before you ask for it. If you don't know what you're talking about, or what you're requesting, how is your reader ever going to know what you're asking for?
So, know exactly what you are raising funds for, how much you need, and how much you will be requesting from donors!
The second thing is to know exactly who you are writing to. Is it to parents, individual businessmen, companies, grants organizations, or previous donors?
Different letters will be more appealing to different types of people. So by knowing who your target audience will be, you will be able to write a more successful fundraiser letter.
You may find that you will need to write a few different letters to accommodate for the different types of audiences that you are writing to...
If that's the case then write your initial letter to one audience and then change and adapt it to be more suitable for your other readers!
And always remember to thank your previous donors, right at the beginning of your letters to them!
For companies and grants organizations you must first do your research and make sure you are addressing your letter to the right department of the company, and to the right person.
The last thing you want is for your letter to be past around from department to department until it finally reaches the right person (likely once it's too late!).
You will also come across more professional and more impressive if you have the right name on your letter.
It will also make the letter more personal which is exactly what you want.
People never really understand the power of a post script. It is such an amazing tool to boost your percentage of positive replies!
Here's why...
The majority of your fundraiser letter will go unread by most of your readers (especially the skimmers), but the PS is almost always read. It's the piece of info that stands out at the end of your letter!
So the point is to use a Postscript, not as it's supposed to be (info that was accidently left out of the main letter body), but rather as a tool that will catch the attention and interest of the reader, and call him to action!