What I did was to have up to five person work on a hit. Many simple hits are done at 0.05$ so it's still not too expansive.

For text submissions, you can enforce validation rules (simple and regex). So you don't have to manually reject non-conforming work. You can also have the Turker fill in their work on a webpage that you control, so validation can be much more advanced. I never went all the way but sometimes I think about it.