1. How often is it used? - We feel that if something gets used once a day, it gets to stay out on the counter. For us, that means the toaster, the bread board and our current loaf of bread, salt and pepper shakers, and a standing container with our most common cooking utensils (mostly wooden spoons).
2. How hard is the appliance to move? - We keep our heavy KitchenAid stand mixer on the counter all the time even though we only use it a few times a month. It's just too hard to move, plus our cupboards aren't really big enough. When we're not using it, we actually use the mixing bowl to store our oven mitts!
We also try to create zones on our counter top: this space is for dirty dishes, this space is for appliances, this space is just for prep (and woe to the person who leaves their library books there), and so on. Once our whole family is in the habit of using and seeing these different zones, we find that it makes the job of keeping things tidy a lot less intimidating.
What's on your counter right now? Any tips for keeping counters clutter-free?