I use the 3-bin wash system whenever possible.
http://scoutmastercg.com/how-to-wash-dishes-when-camping/
Clarification - for short form (cliff's notes) on the pre-soak... if I have a cup with some juice or something in it, I can pour a little water in and swish it around... the idea is if necessary to maximize conservation water and figure that it's your food anyway...

If this isn't appealing, just scrape uneaten food into trash bin for disposal later.
Here’s a short version
Pre-Rinse – each individual uses a little bit of drinking water to rinse out their dishes and utensils,
drinks that rinse water, and then puts their dishes on the wash pile.
Hot Water Wash (First Dish Pan) using as little soap as possible the dishes are washed. The cleanest dishes go first, the dirtiest dishes go last.
Hot Water Rinse (Second Dish Pan) once all the soapy water has been shaken off the dishes they are rinsed in hot water.
Sanitizing Soak (Third Dish Pan) rinsed dishes go into a sanitizing soak.