A little bit of both, I think.
A lot of people have a list of requesters that they monitor every day, in case they post. There are extensions/scripts that will monitor a requesters name and alert you if they post.
When none of those requesters are posting, some people will look on the forum for surveys/batches and work on them as they post. Because decent work disappears quickly, once it's posted on the forum, there's a very small window of opportunity to grab things.
So the key to success is basically just...doing everything you can to find work. You watch for requesters that you like, and you look through Mturk for anything that seems decent, and you watch the forum in case anyone posts something that you've missed.