Pay attention to word count. If your client says 500 words, don't give them 498 or 564. Give them 500 words. Word count is one of the number one things clients complain to me that other writers get wrong. If the client gives you a range, stay within that range. If they give you one number, give them exactly that number. If your office program doesn't have the function built in, use a free word count tool. The purpose of ordering writing services from an outside party is to save time. If your client has to edit your words down or add more words, you haven't completely done what they needed you to do.
Follow instructions exactly. Unless the client mentions that you can be flexible, adhere to the directions they give you. Save the extreme creativity for your own blog or site. When someone hires you to do something a certain way, that is what you should do. It is acceptable to give a client advice and explain why you feel it might be better a certain way. But if they disagree, follow their instructions or pass the gig on to someone else who will. One of the things I get complimented on the most from clients is that I always give them exactly what they asked for. It's not that hard to do. If you don't understand what a client wants from initial directions, ask them questions. Trust me, they will appreciate your effort.
Turn the work in on time every time. If a client needs something by a certain date, make sure you can do it by then before accepting the gig. Writing is not a service that many people can just do randomly at the last minute. If you fail to complete it on time and they need to find someone else who can do it, it makes things more difficult. If they decide to stick with you, even with your tardiness, they may not be pleased whether they say so or not. They'll just use another writer next time. Everyone has emergencies and most clients understand that sort of thing. But making a habit of turning assignments in late will hurt your chances of getting a repeat order.
Proofread -- more than once or twice. By hiring a professional writer (you), your client is trusting that the finished product will be publish-ready. If you fail to check your work for mistakes, you'll likely miss important grammar and spelling issues. This is not indicative of professional work. Always use grammar and spelling checkers for the first check. I also suggest reading your content through aloud, then backward, then taking a break and going over it again one paragraph at a time. Copy and paste each paragraph one at a time by itself to check it. Then, reread it again a couple times as a complete piece. If you have to make any edits, repeat the whole process again. Sometimes edits can create other mistakes you didn't think about. If possible, I suggest also utilizing a writing buddy.
*Disclosure: The blogger received compensation for mentioning some products or services in this post.