You shouldnt trust coin mixers, be very careful with them since they can steal your bitcoins and never send them back to you while they are "mixing them"
That's right,as always in bitcoin, you only have to use trustable sites which have showed to give a good service
Be very careful with mixers. You send money and hope they send it back. Research the heck out of it before using, and start small.
Make sure you really need this service before doing it. Most seem to use them without a true need to be anonymous.