Before you pour sticky substances into a measuring cup, fill with hot water. Dump out the hot water, but don't dry cup.
Next, add your ingredient, such as peanut butter, and watch how easily it comes right out.
Julie Notes: I've never tried this, but my Mom taught me to coat the inside of the cup with vegetable oil first, at least for things like honey or molasses . . . It will slip right out. It works really well too, when you recipe calls for both something sticky as well as some type of oil because you can use the same cup for both ingredients, just make sure you measure the oil part before you do the honey.
I also will use just one cup for as many things as I can so I don't dirty every single measuring cup I own . . . for example: When I make something that calls for a cup of something and also a 1/4 cup of something else, I will usually only use my 1/4 cup and I'll just refill it four times for my full cup measurement. This is especially nice when most of the ingredients are dry.
Sometimes I'll only use two measuring devices for a single recipe, one for dry ingredients and one for wet and I'll only have to wash them each once. Just be sure you read the recipe carefully when you do this so you don't get the math wrong.