Unfortunately I don't think there's a secret. Evaporation and some deposits are really tough to prevent in tiny tanks - you could set up an auto-topoff system (people use these often on reef tanks), but it's probably a bit much for a planted pico.
I have fairly soft water, so I don't get a lot of mineral deposits. I usually run a paper towel around the edge before I top off with distilled each evening.
May not be an option, but if you're getting a lot of evaporation, you could slow it by covering the top of the tank with a piece of acrylic or glass (just leave a little space for airflow).