When passing someone in a bike or multi-use path it's important to yell out "on your left" and slow down. I always make a point to do this but sometimes my voice doesn't carry very well. A simple solution to my problem was investing in this cool bike bell.
When I was little I had a bike bell but I thought it was just a cool accessory — now it's a major piece of safety equipment. The bell's classic ringing sound is loud enough to warn oncomers but isn't obnoxious. It was also incredibly easy to mount on my bike. (If you're not into a bell look for a bike horn instead.)
Since using a bell I've noticed that most people respond kinder to me than when I yell out, perhaps because it comes off as being less aggressive. To play it safe though I use a combination of both if it seems the bell isn't enough.