Apologies it this question has been raised already, but is it possible to break or reassign your own supply lines? I have messed up a bit and would like to change a few things.

Also, tips on how to use supply lines in the best possible way would be appreciated.

You can click on the supply line itself and then hit the red X that shows in the bottom right.

As for tips, that part I've never really worked out for myself either, I usually just hope for the best. One thing I have noticed though, if you set the city at the end of a long chain to need 75/100% it works wonders for pulling food up from your major farm centers.

Thanks! Having efficient supply lines makes quite a difference.