If you have to ride often on a MUP, invest in a decent bell.

Compared to shouting "on your left", a good bell:

• Can be heard from further away.
  
• Is instantly identifiable as an approaching bicycle.
  
• Doesn't startle senior citizens.
  
• Is much easier for those who hear it to identify where the sound is coming from (because of its higher pitch).

Oh, and SLOW DOWN when passing peds. Remember: on MUPs, we're the cars.