Things to avoid include insulting the Grey Wardens or Duncan. Like most romances, it also helps to agree with him whenever you can. When talking to Alistair, make sure to pick a lot of jokes (as long as he isn't the butt of the joke). To romance Alistair, the player should definitely take advantage of gifts. No other romance can get the Warden to the role of Queen of Ferelden.
The only romance for someone who wants to become Queen is Alistair.