I do a run/walk interval and have never SHOUTED anything. What I do as my garmin starts beeping is raise my hand, look over my shoulder and move to the side out of anyone's way and gradually slow down. For me personally it's not a huge deal as (much to my running club's coach's chagrin) my walk isn't that much slower than my run - like less than a minute a mile slower, so it's not a huge shift in pace to those around me as I'm more of a back of the packer kind of girl. Basically make your intentions known in some way.
Other things...
* If you spit, make sure no one's coming directly into your path as you do it. Getting spit on is gross.
* Ditto if you need to blow snot out of your nose.
* At water stations (oh there's a whole novel that can be written on those and how to work them), THANK THE VOLUNTEERS (ditto those stationed along the course). Personally I try to get my cup (and GU packet if applicable) into the trash cans provided to help them out a little. If you're going to throw water on your head (probably not since you wear electronics, but still), I'd also echo the look around you comment. Someone else might not want to get splashed. And if you're throwing your cup on the ground (lots do), try not to throw it on someone.
* If you're passing someone in a very tight spot, saying "passing" is always helpful. If they clearly have their ears plugged up, a gentle touch on the elbow or shoulder can work. But again, it's another reason it's a good idea to avoid plugging the ears imho. (Believe me, I'm not anti-music. I'm just "running in a race is a communal activity done by individuals" when people tune out, it makes it much less of a community. Just my opinion. I know a lot of people disagree.)
Um...most of my other things are more New York Road Runners - and their Young Runners - specific. Like don't sprint ahead, stop, and dash back. OY that's tough to deal with.