First it is important to put pen to paper and make sure that you will be paid enough for your time and labor. This is a brutal business. Be sure that you don't work for any of the middlemen. They take up to 50% of your pay for the pleasure of giving you low paying jobs. Some companies to avoid are Quest, IPSNW, and Homeland. Other "regional companies" higher drug addicts to complete work for a fraction of the going rate. It is amazing how much work people can complete when they need their meth fix. (Do these meth heads really have insurance?)
The name of the game is volume. The problem is that you are so exhausted and overwhelmed that you don't have the time to check that you are being paid for all of the work you have completed and for the correct pay. It is important to check all payments that you receive because you are often shorted.
I don't think anyone will ever say which companies are good to work for since they live with the fear that those companies will find someone to complete the work for less. It happens all of the time. The less that the laborers are paid, the more that is left to line other people's pockets.