Do as much research as possible about a funding source. Don't make telephone solicitations. Ask for printed information, and to be put on their mailing lists. Try to identify someone there with matching interests to your organization who can become an advocate for your group. Often a successful process of negotiation will lead to the submission of a formal proposal for funding which is expected and well-received by the funding source.
Some foundations discourage personal contact and negotiation.
Write them a letter of inquiry to ask about foundation guidelines and briefly outline your proposal, your organization's history and mission, program methods, evaluation criteria, staff information and tax-exempt status. The reply from the funding source will tell you if the lead is hot or cold. Always thank a funding source, even when you are turned down. Don't burn any bridges. If possible, ask the source for a critique of your proposal so you know why you lost out and to also increase your grantwriting learning curve.