And here's a simple slash animation:
The trick is that the arm and hand are animated with a simple three frame animation, and then in code I rotate the sword at a higher frame rate while positioning it in the correct place. The result is a pretty smooth animation with the added benefit that I don't have to manually animate the sword at all thus making it much easier to add different swords and weapons to the game.
I got the idea for this from Terraria, which uses this technique for weapon/tool animations. Some examples can be seen here.