I got annoyed with liquid laundry detergent collecting and drying in the bottom of the cup.
Solution: put a tiny bit of water into the cup before dispensing. Then, after adding the detergent to the load, put a little more in, swirl it, and add that in, too.
Viola, clean cup.
I do that with fabric softener, too. Especially in the dispenser of my front loading machine, it got really gross. I add in some water and it always comes clean. The machine uses water in the process, so it's not like I'm changing the composition of the fabric softener.