mturk is a great place for making little money from microjobs, but the presence of shady employers can drag the whole experience. To protect yourself, keeping track of the reputation of employers is essential.
One great tool to help you stay away from bad guys is this little browser extension, Turkopticon:
Currently, the extension supports Chrome natively, and it can also work on Firefox with greasemonkey add-on. The plugin keeps you informed of the employers’ history by revealing their reviews while you are surfing along the HITs. It also lets you review your employers as well to keep others alert.
If you are new to mturk, you may want to take a look at the mTurk Wiki. This could give you ahead start.