I guess it depends on what you’re hoping to do in the design world. If you are working for someone else (agency, in-house, etc) then you can probably just do it for the experience. It can be refreshing to do something different than your day job.
If you are a freelancer or running your own design shop you have to have an agreement up front – even for pro-bono work. Here are some things you could consider asking for:
Have them ask their board, donors, etc to check with you about their design needs. A lot of organizations have well positioned people helping them along. You never know who might have paying work in need of a designer.
Put your name/link on the footer of websites, add your name as a sponsor for events, and a shout out during events.
Depending on what you’re designing for you might score tickets to the event, gala, concert, fundraising dinner, gallery opening, beers/coffee with someone at the org you admire, etc. Insist on coming and bringing a friend or two.
Depending on your aspirations you could probably skip this, but bigger projects generally need publicity when they are done. Insist on your name/contribution be included in the release.
Rights to your Work
Generally when you are contracted for work all the rights are given to the client upon payment. Ensure you get the rights to display the work in your portfolio and anywhere else that makes sense. Good work could be entered into various contests and exhibitions – but you need to have the legal rights to submit it. Great work could win you an award, as long as you can enter.
Don’t feel bad asking up front for these things. Design is a big deal, and good design is a labor of love. If they don’t feel any desire to help you, I would turn down the work. There are people who don’t value good design and just want it done for free. These people tend to be miserable to work with.