How can I make a game of Memoir '44 last longer?
Do you play each scenario twice (switching side each time)?

This has a double advantage:
First, you set the scenario up once every other game instead of once each game.
Second, as the scenarios are based on true events, they are not always 50/50 balanced. The idea by playing on both sides is to get a global score (sum the medals up to get an overall winner).

If you feel it's still too short you might want to investigate the Breakthrough mode which can be enhanced by the Winter Wars Expansion