Conventional economics says that we should give as little as possible while taking as much as possible. Further, we should try to appear to be giving a lot while appearing to take little. Yuck.
I prefer transparency and sacred economics.
If you just put up a "DONATE" button, people will mostly ignore it. We're used to the idea that most of the internet is free. Retraining your users is hard, so be prepared to put some effort in to it.
The people who use your site and benefit from it can understand that it costs money to run, and that you have a need to make a living. You can explain why it's important to you to not to pick the other forms of revenue. For me, I might write:
Much of the internet is offered for free. Usually that means that you look at ads while you browse the web. I don't like ads. Whatever value I get from a web site is reduced by the ads I'm forced to view. I don't want to compromise the value of this site by littering it with ads.
You can donate to the costs for this site and my salary, which are detailed here. Your donation keeps the site running, and shows me that you want me to continue spending my days making this site better.
How much should you pay? Choose an amount you feel good about. When you choose the right gift, in this or any situation, you'll know it by a feeling of rightness, lightness, and clarity.
Perhaps you have a suspicious attitude toward me for requesting payment, imagining perhaps that all this "gift" talk is a stratagem to get more money. I don't want to sow that kind of cynicism. Instead, you could pay with a gift to someone else.
I took a lot of this from http://www.ascentofhumanity.com/voluntary-audiovideo-payment.php