I usually post an announcement on the forum, post it on our twitter page, send out an email, post it in the "Whats going on?" / "News Broadcast" sections of advert forums & in my advert threads.
Sometimes it helps to announce in advance that you will be opening an exciting contest on x date, without giving the details. That way your members will be looking forward to it and eager to know what kind of contest it will be. But only do that if you are confident that they will be impressed by the contest, you don't want to disappoint them.
