Answer:
The Number One tip - gays and lesbians are just people. Writing gay romance, for example, is no different from writing straight romance - you just have two people of the same gender instead of one of each.
Gay and lesbian stories depend on good characters most of all! If you've got a vivid character, the reader is not going to care what sexual orientation they have - and if you've got a vivid character, your story will fall into place as you imagine how that character reacts to the world around them.
Often, but not always, a gay and lesbian story will include some sort of political message about intolerance - either explicit or implicit.