I like Low Tech Magazine's use of dithering to reduce bandwidth and storage consumption. I'd played around with dithering before and come up with a set of options I really liked... but I didn't record them anywhere, so now it's lost.
For the grayscale images in this wiki, I'm using:
convert in.jpg -resize 640x480 -colorspace Gray -separate -average -ordered-dither o8x8,6 out.png
This command usually turns the greater-than 500kb JPEGs from my phone's camera into PNGs smaller than 30kb.
For the color images in this wiki, aside from the images on the the camera pages (as those are meant to accurately represent the output of their respective cameras), I'm using:
convert in.jpg -resize 640x480 -ordered-dither o8x8,3 out.png