I added a sand pit after the initial setup using play sand. Make sure to stir it every now and then while washing. For adding it without having it cloud I used a plastic cup. If you tip the cup into the water slowly all the sand will stay inside it, making it easy to pour out where you want it, without it clouding up. Worked great for a 10 gallon, for a bigger tank Agnes' 2L bottle idea sounds more practical.
-Rob
-Rob