I only started updating my wardrobe about 6 to 8 months ago after finished grad school and entering the real world. Needless to say, I have made numerous mistakes and most of them have been rather COSTLY. Live and learn I guess.
1. Trying to buy a new wardrobe in one sitting instead of learning about mens fashion and waiting for deals. When I initially got interested in clothes and shoes, it was a real high and I want to make some immediate changes to look and feel better. Unfortunately, this meant that I went out and purchased a slew of crap without thinking about brands, quality, proper fit, etc. I also ended up with a ton of garbage and had to sell or throw out because I just went out of a shopping binge without thinking about it.
2. Another mistake I made was not realizing that quality is greater than quantity. During my initial shopping spree, I went out and grabbed a bunch of shit from Banana Republic, GAP, etc, and I simply didn't care about construction, materials, etc. It took me a while to realize that it really is better to have 2 or 3 really quality sweaters than 10 average mall brand sweaters.
3. All shoes are not made equal. Oh boy, I really didn't pay attention to shoes during my first attempt at updating my wardrobe. I had a few shoes and thought that was good enough, and even when I purchased shoes it was just cheap crap. But shoes really do make an outfit, and I'm slowly learning about stitching, leather, etc and how all that matters. Unfortunately, I have about 8 or so dress shoes and only 3 or so are really quality, so I'm still working on this part.