as a freelance illustrator and someone who's done  and still do comics like what you're describing .
i agree with Tiari ... make sure you have EVERYTHING . in writing ..  draw up a quotation . on your letter head if you have one  . , my quotes usually have a ''terms and conditions ' disclaimer on them
i do not proceed with any work until written approval from the customer..  also i'd advise to keep all correspondence  between you and the customer .. whether it be via email or  hard copy . until you get paid  or till the end of time lol .. theres nuthing worse than  the ... ''but you said this and you didnt do it scenerio''   and lastly ... ask for a deposit ..

just my 2cents worth :)