From Doktor Frank, quoting Hemingway, or Roald Dahl, or somebody.

The best way is always to stop when you are going good and when you know what will happen next. If you do that every day you will never be stuck. Always stop while you are going good and don’t think about it or worry about it until you start to write the next day. That way your subconscious will work on it all the time. But if you think about it consciously or worry about it you will kill it and your brain will be tired before you start.

Outstanding advice, but there is a tradeoff.  Being on a roll means being connected to what you are writing right now.  And that connection can lapse after time passes, even just overnight.  Material which was gold last night can turn in to lead the next morning.  A better bridge is to conclude the session with rapid, sketchy, outliny writing and start the next day by fleshing out the overhang to re-establish the flow.