There are services called tumblers which take bitcoins from multiple people (often multiple sources per person); mixes them up with bitcoins from lots of other

There are services called tumblers which take bitcoins from multiple people (often multiple sources per person); mixes them up with bitcoins from lots of other people, transfers them about a lot randomly etc, then at the end spits out a series of transactions which add up to not quite the amount you submitted (different by a random amount). Each of these transactions can be traced, but it can be difficult to track which output bitcoins relate to which input bitcoins.