Step 9: Other tricks.
1) Work with a smaller palette. WIth one particular image, I could not get my combination of two Post-It sets to generate a halfway decent mosaic. The problem was that there were three reddish colors, with everything else either a yellow, green, or blue. I created a palette with 7 colors, eliminating the reddish, and suddenly everything looked much better. Sometimes less is more, I guess.
2) Factor in the color of your wall. At the school at work, we have walls of pale blue, green, or tan. Not sure how that will look in the end, as I've never tried it myself, but it's an idea.
3) Use a marker to colorize some regular Post-Its. A black marker on some yellow Post-Its may give you just the contrast that you need. Experiment.
4) Try different sets of Post-Its. This would seem obvious, but if you're trying to work with what you have on hand, you may not get the results that you really want.
5) Ditch the Post-Its entirely and work in a different medium. This technique will work with any square object, so little ceramic tiles are not out of the question.
6) Smaller pixels (shown below). Either cut down some Post-Its or use pieces of paper. Every doubling in width (or height) means four times the amount of pixels, so keep that in mind.
Most of all, don't hesitate to play around with the software. Change your resize settings, or mess around with saved palettes until something you like comes about!