Tell them the anticipated impressions such a sponsorship will yield. Impressions are calculated by finding the total number of times a sponsor's name will be seen or heard. For instance, say your event is expected to draw 100 people. Make sure sponsor benefits are easily found in your letter and they're easy to understand. Consider using bullet points to make the benefits stand out.
Hand sign each letter.
Finally, include a chart or brochure that details sponsor benefits at each level. If this is the second time your organization is hosting the event, include a flyer that lists the sponsors and describes the audience from the previous time. Don't be afraid to call potential sponsors to find out their thoughts on sponsorship. After receiving your letter, some companies will call you to say they're interested in sponsoring. Most will not.