Some years ago, I noted that nearly 60% of households in the United States give to charity in a year yet just over 10% of people who have a will include a charitable gift (and about half of adults age 55 and above don't even have a will).
However, a few different studies have found that people would consider making a planned gift if they were asked. Ask long-term donors to include your charity in a will, as a beneficiary of a life insurance policy or in some other form of planned gift.
If someone says it is too expensive to write a will, this site might help them. I did this for myself in less than 15 minutes and named two charities for specific amounts. I had the option to leave a percentage instead of an amount of my choice. The hardest part was looking up the address of one of the charities I selected. Now I have a PDF to print, go over with my spouse and kids, then sign with two witnesses. That's it.
Easy-peasy as my grandmother would say.