I was just emailed this link: Writing Gay Characters. It's got some amusing covers from the early days of the modern paperback, and it's a fine list of ways writers can screw up.
But I don't think it's what it intends to be, because I don't think you can teach people how to write gay characters well. Either you understand that all people are people, or you think people like you are people and everyone else is inferior. That applies to more than the cliché of the straight white male: There are gay bigots, just as there are female sexists and black racists. This may be the most important lesson in life: people are people.
I only noted the sexuality of a couple of characters in my first novel, Cats Have No Lord, but I'll happily pull a J. K. Rowling and retcon Thraas: He's gay. I included a gay male couple in my second novel, Witch Blood, because it was about a small community, and I couldn't believe a small community would not have gay folks in it. No matter whose figures you use for what percentage of humanity is gay, if you have ten or more folks in one place, odds are good that at least one of them is gay.
I grew up in a fairly homophobic time, but Dad had been in the Merchant Marine, and he told me when I was beginning to understand that human sexuality is complex that every ship has gay sailors who do their job just like anyone else. Dad's principle of life was one that true conservatives and communists agree with: Live and let live. It's always worked for me.
I've only written one story that was about being gay: Secret Identity. Otherwise, I just try to write the world I know, which includes gay folks.