Don't know if it's the "best" way but... Dawn dishwashing detergent works well on sap. Right out of the bottle, drop or two on sap, let sit minute or so, then wet rag. Will take some rubbing and maybe couple of tries but actuall works pretty well.
Due to depreciation the above opinion is now worth less than what you paid for it.
Everyone is good at something... I happen to be a good bad example.
2016 Forester 3011DSF