don't clean them up? make something artsy out of how shitty your scans are

adjust brightness/contrast

Depends on a few things, how "clean" your existing layout is, how detailed or time consuming your sketches are, how much you want to include them...

My first thought was just to grab a roll of white trace paper and find a light table and re-trace the ones that are the most trouble. Depends on how easy they are to reproduce.

My other thought was to maybe create a new layer on top of your sketch layer, then use that polygon selection tool thingy to outline around the outside (and then inside) your sketch and just fill the area with white (or whatever background color you have. Then maybe use the blend or smudge tools or something. Works ok with sketches that are solid black lines, if there's shading involved, I have no idea how to get rid of the graph lines under the gray shading, unless you can go through with like a pencil tool and the eyedropper and manually change each pixel where the line is, but that sounds like a pain.

Honestly, most of my early process sketches weren't that good or detailed. Pretty much all the sketches that I plan for my upcoming portfolio I'll be re-drawing or drawing from scratch on white trace and scanning in...

Uhh post some work.

To make the background all white (or any color, for that matter), use the "select by color" function and click on the background (or white area). Then play around with the fuzziness so that you pick up as much white area as possible without picking up any of your sketch lines (usually about 40-50 for a B+W hand sketch). Once you have that area selected, just fill the area with a solid color. This should give you a pretty clean background with minimal retouching necessary. Make sure you are working with a high-quality scan though.

select black... right click similar.... right click cut/make new layer..... bam.......