I did add qualifications, although I forget which ones, and results were shaky. What have your experiences been with qualifications, and which ones did you use?

Not who you were replying to, but on mTurk 10,000 approved HITs with a 99% approval rating or higher will probably be your best bet. If you are going to put up a lot of HITs, especially if your HITs require special skills, you might want to put up some tasks and assign a special qual to the top, say 10 or 20% of the workers. Then you'd have a more limited group of workers, but ones that you know will do well. (Usually qualification tasks require workers to complete 10-20 of the HITs to get a good idea of the quality of work.) You might sacrifice a little bit of speed, but you'll get better work. Plus, if your work pays decently by mTurk standards, anyone who has the qualification will be on the lookout for your tasks.