For occasional surface cleaning we have found that following products work very well along with a quick rinse from your garden hose:
Simple Green - A good environmentally friendly cleaner
The Works: Basic Bathroom Cleaner with Bleach - Our production facility has been testing a number of regular household cleaners and have found that this one works surprisingly well on our products and is obviously a great value!
Mr. Clean Magic Eraser - Great for our White products
Armor All - Great to bring out a shine on our black products
For a deeper clean, we recommend the use of a high power pressure washer. As polyethylene is slightly porous, small dirt particles can become trapped over time and a power washer, used up good and close will be able to really blast the dirt away. Since our products are molded entirely of the polyethylene and do not have any type of finish there is no risk of damaging the planter when cleaning with the power washer. It may take some time to remove all of the dirt and stains, but with the pressure washer on the highest setting, you'll find that the results are fantastic!
Here is a photo of my own personal white planters that I have had for 5 years that I cleaned up with my power washer this spring. As you can see they came out looking just like new! (I have also attached a video so that you can view the cleaning process in action.
Another handy tip: when re-planting I found I was getting dirt all over the rim and in the pores again so while planting I took some kitchen plastic wrap and put it around the ledges to keep them clean. Once I was done filling and planting I took the plastic wrap away and had perfectly clean and beautiful planters!