Explain what you are going to be teaching before you even start. Tell the preschool students it is time to learn how to wash hands, or it is time to learn the ABCs. Giving students an idea of what they are about to learn is one way of making them comfortable with the technique that is to follow.
Choose one technique to start with, and present it. For instance, for washing hands, choose a song that goes along with a hand-washing demonstration, and show it to the students. Don't introduce any other method for teaching the technique at first. Stick with the song and the demonstration, or whatever the method is you've chosen to explain the technique.
Repeat the method you've chosen to explain the technique a couple of times a day, and do so every day until the students can do it along with you.
Introduce another method for teaching the technique. If you have taught the hand-washing technique through a song and demonstration, add a colorful poster that explains the steps. This second method makes the technique even more ingrained in the minds of the preschoolers and gives them a way to remember the technique you've just taught them.
Remind them of the technique every few days, until you are sure they have remembered it. Keep in mind that you might still need to refer back to your teaching method over the coming weeks and months, to reiterate the lesson the students have learned.