1. Have you worn it in the past year?
If the answer to this question is no, it probably needs to go. The exception to this rule would be high end designer items, cocktail dresses, etc. But, try your best not to make up exceptions. I'm 99% sure your future son/daughter doesn't want your random shirt/dress from Banana Republic that you haven't worn in 10 years. He/She will buy their own.
2. Does it fit?
I don't know why we buy or keep things that don't fit. I'm so guilty of this. But we all know if it doesn't fit right we aren't going to wear it, and even if we do, we aren't going to be happy about it. So, let it go. OR get it tailored. Tailoring is so important and probably deserves its own post or at least bullet point. We are all a million different sizes and shapes, to think that we can be perfectly defined by a 2, 4, 6, 8 is crazy. Find a tailor and make he/she your BFFL.
3. Does it make you feel good?
If you look good, you feel good. Plain and simple. My goal is to love every single piece of clothing in my closet. Obviously what I love is going to change with the seasons/trends, but for the most part I want to feel excited about putting on everything in there. I am not there, but I want to be. We all have those things that we put on and just don't like, but we wear them anyway because its there and we don't want to wear the same thing we wore yesterday. Whatever those things are in your closet, get rid of them. They are bringing you down. Build a wardrobe that you feel good in.
Good luck and godspeed!
If you need a professional, feel free to contact me, firstname.lastname@example.org