If you take away only one thing from this page, remember this: There is a waiver for almost anything. (This is especially true if you're heading into the Guard, of course). To be sure, there are some medical issues that you cannot overcome, but you will do well to make researching waivers a habit whenever you hear "no." It might always work out, but it certainly won't hurt to ask.
Flight docs are generally knowledgeable about which conditions are waiverable and which aren't, but you should make it a point to research the guidelines for waivers for any medical issue you know of. (Review AFI 48-123 Attachment 4 thoroughly to try to identify anything in your medical history that might be a problem.) The Waiver Guide should be your first stop for research. Because it can be written in 'medicalese' you might want to consult the family physician before your Flying Class physical to see if you fall within the guidelines for a waiver. Only the opinion of the flight doc counts, of course, but if you have documentation showing you were previously examined and fell within the guidelines and bring that up, you might at least get a re-examination or second opinion.
Most flight docs are happy to help you - they're not trying to arbitrarily keep you from flying duty. We're working to bring a flight doc on board to provide more specific tips on getting the waiver you need, but in the meantime users who have received waivers have shared their information below!