When I started thinking about putting together an article, the work of Fred Van Lente sprung to mind as probably the best example of script writing that I currently work with. Comics scripts share their roots with playwriting/screenwriting, but have some specific differences, as you'll see.
Common mistakes are probably the result of not seeing other writers' form a lot of comics freelancers tend to work in a vacuum. Often the editor and letterer have to comb through a malformed script and tidy it up before the next step in the creative process. Above all else, simplicity and specificity are what make for the best form. E prepared to see a lot of differently formatted scripts from writers if you want to letter comics.