Do you use liquid laundry soap? How about those clever containers that have a spout and cup? It's what I use. I love the easy convenience of pressing a button to get the soap. I do not like how horribly messy and sticky the cup gets, and the waste of soap left in the cup.
A couple weeks ago my mother in law was visiting. I told her about my problem with the cup, and she said "Just toss it in the wash". Really? I had thought of this before, but I was sure it would melt the cup, or break it into tiny pieces. Plastic just does not go in the washing machine. Well, with everything, I took her advice under consideration, and tossed it into the wash.
It worked. The clothes get the extra soap from the cup, thereby not wasting it, and the cup gets clean.