How to decide what goes
Short answer: Three questions are enough: have you worn it within a year, does it fit, and does it match at least five other garments. Two noes mean out.
The three questions
Wear within a year shows whether you actually choose the garment. Fit shows whether you can. Matching shows whether the garment does work in the wardrobe. Together they cover almost everything.
Why matching counts
A garment that can only be worn with one other is rarely picked. That is not the garment's fault, but it makes it dead weight in the wardrobe. See bamboo clothing for women.
The exceptions
Formal clothing is worn a few times a year and should not be judged by the same measure. The same goes for outer layers for particular weather. See bamboo clothing for men and bamboo underwear for women.
Frequently asked questions
What if I am unsure
Put it in a box for three months. Doubt is an answer in itself.
Should you keep something in case it comes back into fashion
Rarely. It takes up space for years and does not fit when it happens anyway.
What about clothes that look like new
If you have not worn them in two years, you will not wear them next year either.