you should make your bitcoin anonymous from the start. which means you should buy it with cash from someone and place it in a new address.
this way it will have no links to you, because buying from any sort of exchange or using credit card would link it to you.
If you use it at some point, you've now created a link between you and the coins. So you still aren't anonymous.
I think we need to define what you regard as anonymous and the situation you use your coins in. You can still keep your identity secret but not if you post you public info along with the transaction or it is linked somehow.