Step 2 - Sanding
After the container is clean, we need to roughen up the surface. This is the step most often skipped when painting, but is critical in allowing the paint to adhere. The best method for this is to use either steel wool or very fine sandpaper.
I usually use a medium-fine grit sandpaper (in the 600 grit range), and gently scuff the entire outside surface of the container, paying particular attention to the corners and latches, and any areas that will experience increased wear. The goal here is to turn that factory glossy surface into a rough paint-gripping one, and you'll know when you've done enough as the container will take on a dull appearance - there should be no shiny spots.
The sanding process should only take a couple of minutes; take your time and be sure you don't miss any spots. There's no need to be too aggressive, you just need to introduce some very slight scratches so that the paint has something to hold on to.
Step 3 - Cleaning
Once you've fully roughed up the surface, you can move on to the critical cleaning step. Having a rough surface really helps give the paint something to grab hold of, but if there's dirt, oil or other contaminants we're not going to have a lasting paint job so you need to make sure each of the containers gets a thorough clean before you paint.
As previously mentioned, some people put containers in the dishwasher to clean them. This works fine and i've used this approach myself on a number of occasions, but I usually prefer to just give the containers a good scrub in some soapy water, followed by a rinse, and then put them aside in a nice warm spot to dry.
It's important to dry the containers indoors, such as a garage, to minimize dust and debris from settling on the surface. Usually overnight is sufficient, but depending on your abmient temperature and humidity this may vary.
Just be very sure your containers are completely dry before proceeding on to the painting step.