You can use an auger and/or remove/clean the trap. I'd try a hand auger rather than the more aggressive motorized auger which could damage plumbing.
My Son put wax crayons down the sink many years ago. The sink seems to
>constantly get clogged up and drains slow. We have had Mr Muscle sink and
>plughole unblocker down it a few times but it still does it. Any idea how
>to clear it up?