Attention to uniformity: If you wear a rugby polo shirt, also wear casual pants or jeans, not dressy pants. Conversely, if you put on a shirt, put on designer jeans or wool pants. Breaking the uniformity to play with contrasts is possible but more difficult to master and wear. At first, just have a defined and uniform style.
Don't go shopping alone: Salespeople will tell you anything, as long as you buy their items. So it's easy to blow your budget and leave with things you'll never wear. I am well placed to know it: in every man makeover, I keep the salesman busy and I take care of choosing the pieces without listening to him. And it's not a question of the level of the store. Some Dior salespeople told me that a jacket that was objectively much too small for me was my size because "that's the way it was worn".
Take a few risks: It's better to be slightly overdressed than underdressed. One will always notice someone who is slightly better dressed than everyone else. Be careful however not to dress better than the person who receives, your boss or the groom... question of tact. The whole thing is to do it by finding his style, so that it sticks with your personality.
Don't underestimate the details: The devil is in the details. Details are often the first thing you look at in an outfit. So pay special attention to them.