There are 2 things you can do.
1) Cut the stems about in half to a bud facing up. This will encourage a new branch to grow up.
2) Put a small oscillating fan near it, within 8' or so, and set on low. This will cause the stems to move a bit which will cause them to get stiffer and not droop over. It's the lack of movement that makes the stems weak and droopy. Sort of like never moving your arm and then expect to be able to lift your weight with it to do chin-ups.
You can even do both.
