I think the common mistake that a woman does is selecting a dress that does not suit her looks or match her personality. Keep in mind the fashion trends but do not select a dress that is not suited with your persona. What might look good on your friend might not be suited for you. So do not go for common trends. Rather be introspective. Identify your own trend which would suit you!
Next keep in mind the occasion. Do not wear a loud dress at a mourning party or a somber one at a happy event. Know what will go with all occasion and wear those clothes. If you are not sure check out in the internet or consult any fashion consultant. Be careful about fitting. True you might not wear the branded clothes but you can always wear something which fits well. So pay attention to the fitting and find clothes which fit you perfectly.
Labels do not matter but finish does. So buy clothes which have a great finish- after all a worn out button on an expensive dress will make it look shabby! Check out the accessories. Even a gorgeous dress will go waste if you accessories are wrong! So make sure you wear great shoes and have proper bags. A mismatched shoe or a bag can mar your complete look and so do be careful.
Finally be confident. Fashion mistakes do happen and we all do make that faux pass. However the important thing is to be confident about it and carry it off well!