I do at least one ride a week in a rural area with lots of dogs... Here's my experience:

For the most part, a firm deep voice saying "no" or "go home" works. Make your voice as deep and loud as you can. The deeper your voice, the better they respond in my experience (worked on training my dogs as well). This typically works on the bored/yappy dog, not the aggressive dog.

There's one pit bull mix that doesn't get the message and has bitten more than one cyclist in our local club (lucky for us, the cyclist is also a lawyer and has written some letters... the dog's been absent the last few rides). For those dogs, I carry pepper spray designed for runners/cyclists (I got mine at Academy Sports & Outdoors--just fyi since it took me awhile to find it). It has a range of 12-15' and a clip. Clip it up under my shorts or put it in my bento box. I haven't needed it so far, but several folks I ride with use theirs before I got to the dog.