Prominently and repeatedly make your ask. In fact, some of the best direct mail fundraising letters not only make the ask several times in the letter, but also in the outer envelope teaser and the P.S.
While you’re at it, make sure you’re asking for something specific. Specific requests outperform generic asks all the time. Rather than asking donors to “give what they can,” customize the ask to each donor. Based on their past giving behavior, that allows you to ask a consistent $25 donor for a gift of $35, $50 or $100, but ask a $500 donor for $750, $1,000, or to “be as generous as you can today.”