so what program you are using?
i usually open a new layer, then i start doin' cover-up's over the section i did wrong, then i merge the layers.
usually i'll separate the file in few layers. Outline, Shading, Color & Finally BG as the lowest. Outline will always stays on top to avoid hassle when i need to redo any part that fucked up.
i don't really know what to say
don't rush, and you'll get it right. there's one time when i was rushing an assignment that need to pass up the next day, the comp suddenly
GONE BLUE. (i use my office's old pc while i waiting for my new macbook. and worst of all, i have a bad habit of not saving my file in the middle of my work.) at that moment, i was real pissed that if i happens to met Bill any day, i swear to god i' gonna rip his balls out. yeah. i'm that pissed.