Donations, don't be afraid
There is honor in holding out that rice bowl. For many people asking for help goes against their sensibility. Yet, as a developer of Open Source software you must overcome this obstacle. Like the Buddhist monk with his rice bowl, the church with its donation box, or the Public Radio station with its pledge drive you must also ask for support.
Also, like the charitable institutions mentioned above you can not be meek about asking for help. If you are quiet with your donation requests, they will likely be ignored. The simple truth is that most WILL NOT DONATE. Here at DSL, we ask loudly and repeatedly for donations, yet I estimate that fewer than one percent of our user base will contribute to our project. When we asked quietly, with a little donate button hidden deep in the site we were lucky to get $5 a week. Today, we do much better than that, but it is still considerably lower than our revenue from advertisement -- which should show you how critical advertisement is.
Make no attempts to water down your requests for financial support. You should instead remind your user base that you make the fruit of your labor available for free, and that in turn you appreciate your software's fans to give a little back. If you can get a small percent of your user base to give a small amount, the donations will add up. Here at DSL we have a goal of having an average of 1% of our user base give a one time donation of $1. We figured that if we could accomplish this we would be set; we are blessed with large user base.
As it breaks down, we still have much less than 1% who are donating, but the average donation is more like $5. We currently get a bit less than one in every thousand to donate, though it looks like the percentage is gradually improving.
Be sure to offer multiple ways for people to donate. Most contributors to DSL prefer using Paypal, but others would rather use their credit card, or send a check by mail.