What are some good reasons to cancel a membership at let's say a clothing company? I've heard that the company is rather awful to customers who want to cancel and drill them with questions. I'd like to avoid that as much as possible by giving them a good reason and, of course, being as polite as possible. Any thoughts?

I've joined the nudist community.

If it was me , I would just say I don't have the money any longer for their product. Then make sure they can't get to your money (contact credit card etc or stop whatever form of payment you're using). 

Most places ask you to fill out a questionnaire when you stop services because they use it for marketing purposes. You never have to answer anything you don't want to. Just tell them if they send anything else it won't be paid for.

Just tell them you want to cancel your membership.  If they ask why, be honest with them and firm that you aren't interested in doing business with them anymore.  If you don't give them a chance, they can't try to bully you into staying.  

Just give whatever your actual reason is. If they drill you or act disrespectful, just ask to speak with a supervisor. Tell the your reason and tell them the way you were just treated gave you more reason not to stay. Tell them you are canceling and that you would like a letter or email confirming your cancelation. Be sure to ask who you re talking to and write the name down along with the time and date you talked to them. Also ask for a direct number to reach them should you need to do a follow up.

You dont have to 'explain' yourself other than to say you don't want to remain a member any longer.  You want to cancel your membership because you've changed your mind about continuing as a member. . Make sure you read their cancellation policy thoroughly. If they pester you about it .. Repeat "I don't want it" .. Over and over until they 'get it'. The best way to cancel any membership is to do it in writing (and keep a copy) .. If your membership fees are being paid by a credit card, you can even contact your credit card company and advise them that this company is giving you a hard time about cancellation of a membership and you do not authorize any more payments to them. That way you will have a physical record of your giving them ample notice to cancel your membership.