One of the common themes in laundromat etiquette, you'll notice, is to be considerate of others: their time, their space, their belongings. To ensure good laundry karma, and score points with the laundromat regulars for not wasting anyone's time, sort your laundry before you go to the laundromat.
Most people begin with sorting by color, which leaves you with, potentially, four piles: white clothes, light clothes, dark clothes and any articles of clothing that may bleed (such as red socks or dark denim). Separate delicates from more substantial clothing. And depending on how thorough you like to be, also sort by similar fabrics and similar textures (such as a towels-only load), by level of dirtiness (such as workout-gear only) or by weight (keeping in mind that not all articles of clothing are equal; bulky clothing such as jeans and sweaters, for instance, weigh more than cotton T-shirts or anything made of polyester, by volume.). As you sort articles of clothing, check pockets for any stowaways, such as money or jewelry.
Similarly, make the best use of everyone's time by removing all your clothes from the dryer and then folding them — that's why there are folding tables.