Overcast days are the best times to take pictures of waterfalls. Here are a few points/tips:

• Get closer. Get in the water. Wear sandals and bring a tripod.
• Avoid including top tip of the waterfall in the frame.
  
• Take a low angle.
  
• Bring something to dry the lens.
• Bring filters, such as polarizers and netural density filters.
• Take shots using various shutter speeds, starting from half a second and take it from there.
  

I also recommend this short video tutorial on taking pictures of flowing water, and this podcast tutorial.

This was one of the first few pictures my friend took with her first digital SLR. What fun.