Other than using a mixer, people can use an exchange or a casino to mix their bitcoin.
Looking at it at another perspective, if your bitcoin has gone through a casino or an exchange in the past, there could already be some taints.
Well that is kind of a problem see, what happens if someone is running a big illegal ring and comes into contact with hundreds of thousands of bitcoins? All of those bitcoins can be traced back to criminal transactions and then be seized... This could be any fraction of a bitcoin that comes into your possesion, aside from newly minted blocks.