I use one of those cheapie saddlebag style panniers that has two side connected with a piece of fabric on the top. It's filled to the brim on both sides with clothes, shoes, lunch, spare jackets, etc., etc. in the winter. But in the summer, I can get away with a trunk bag because summer clothes pack so small. Back in the student days, I did use a milk crate and then a wicker basket on the rear rack. Bad for balance but great for access!