Pay attention to every detail of your work at all times. Good consulting means sending in work that is completely finished with few to no errors. No client wants a provider who does not follow directions properly or give accurate advice.
Not every professional is an expert at giving flawless results. It takes time to develop a natural attention to detail, and even so, there are plenty of times when professionals make errors.
If you create a set of rules for yourself, and follow each one faithfully, you will improve your work output.
One rule is to double check your work thoroughly before submitting it. If you are writing some documents, make sure you edit every word carefully. If you cannot provide certain pieces of advice, do not make up a sloppy answer on the spot. Your client will not make a referral for you if you give wrong advice. Paying attention to everything you do is an important skill as a consultant.