Make sure you are always checking over the spelling and grammar before sending a piece to print or hitting send on an email. While a misused comma or other punctuation marks may not seem like a major problem, there are plenty of people out there that will notice common issues like that, and ignore the rest of the project. For example, if you are distributing leaflets as part of your ad campaign, and if the leaflet design has many spelling mistakes in the text, it will backfire. Customers may not take those mistakes kindly, and simply deem your business as unprofessional due to tiny spelling mistakes.
What can you do to avoid this problem? It might seem unlikely that you would miss an obvious error, however it can be very easy to overlook a typo even through multiple rounds of edits. So, get a second pair of eyes on your work; have your coworkers look through your copy and hopefully they will catch any proofreading errors you might have missed the first time! Read, Re-Read and Read Again.