A lot of my sketches are oh-so messy. It's taken a while for me to get them cleaner without a lot of work or erasing. You can try sketching with a pen from time to time to get yourself used to planning your stokes and getting into the mind set of every-stroke-counts. I didn't like drawing with pen a long time ago, but now I like it and find it very beneficial to my growth as an artist. Sometimes it is just nice to let go and not worry about if it is messy or not, especially if I will just transfer the drawing to watercolour paper afterwards.