You can make your Béchamel sauce up to a day ahead of time. Cool it to room temperature, then place in a lidded container, cover and store in the refrigerator. Once ready to heat, remove from the refrigerator and uncover. Remove the skin graft (if you don't, the skin graft will end up as lumps in the sauce, it does not cook down and smooth out). Gently reheat in a wide saucepan over medium-low heat until piping hot all the way through, while constantly whisking. You may need to add some extra milk to thin it out and get it fully smooth again.