The primary means by which clothes are cleaned in any washer is by the movement of the clothes through the water.  With that in mind, if you want to get your clothes clean, you must make sure there is enough room in the washer for the clothes to move through the water.  Overloaded washers do NOT get clothes clean.  It's best to never fill the tub more than 3/4 full.

In a dryer, it is the combination of hot air being forced through moving clothes that ultimately gets your clothes dry.  With that in mind, the fewer items you place in a dryer the quicker they will dry. 

In a laundromat, you never know what the person before you may have left in the washer or dryer.  Before you put your clothes in any equipment, make sure that you check to make sure the cylinders are empty.