Here are some tips for how to take scanned comic art and clean it up for inclusion in a CBZ collection, using Photoshop.
Step 1: Scan
When scanning, I like to scan at a high enough resolution that I can see individual halftone dots, so I know I'm not losing too much detail. For the project I'm working on, that means scanning at 600dpi. Later, we'll resize the image so that those dots join together into areas of smooth color, but we want the starting artwork to be sharp.
It's sometimes tricky to get the pages to lie flat without taking apart the source. I've been using a flat clipboard and a scanner that I can place on top of the original.
Step 2: Clean Up the Color With Levels
Use the Image > Adjustments > Levels... command. Select the white eyedropper and choose a color to treat as white. In comics, the gutters between panels are usually ideal.
While you're in levels, you can adjust the overall brightness/darkness of the image. This old page didn't have very solid blacks, so I've moved the black point to the right.
Step 3: Clean Up the Gutters With the Magic Wand
Step 4: Use Perspective Crop to Straighten
Choose the Crop tool and make sure that the Perspective option is on. This will allow you to drag each corner anchor around independently.
Drag each corner anchor to a corner of the art on the page.
You probably don't actually want to crop to this area, because things may be sticking out (in my example, some word balloons protrude outside of the frames). Grab the center anchors and drag them out to the edges of the document. The skewed angles will stay the same as the center of the crop area moves.
Step 5: Resize Art and Copy Into a Fresh Page
We'd like all of the pages in our CBZ to be the same size, so make a new empty page the size of the original page (in my case 8.5" x 11"), and copy the art into this page.
Save the file and start working on the next page!