I got this tip from a friend so I cannot claim credit. Go to the store to buy a bottle of foaming soap (I got soft soap, it was the cheapest). After enjoying the soap that's in there replace it with your own mixture.
This is especially great for kids! For either getting them excited about washing their hands or to save on excessive soap usage for the enthusiastic hand washer.
You can use a liquid baby soap and then rock the bottle back and forth once (don't shake!!!). I use the regular creamy soap but you have to rock the bottle back and forth for about two minutes to get the soap to disolve properly into the water. Otherwise it will clog the pump. I also suggest periodically (maybe once a week) rocking the bottle back and forth once to make sure the soap doesn't all settle at the bottom.
This has really cut down on my soap expense!