The mistake most people make is buying tailoring a size up for comfort, then wondering why the whole thing hangs like it belongs to someone else. A blazer or trouser needs to sit close through the shoulder and waist to look like anything at all. Buy for the narrowest point that fits and let the rest be adjusted, because a good shape with room to take in beats a baggy one you can't do anything with.
Tailoring isn't the right choice for everything, and it's worth saying so. If you need to run for a bus, bend down to lift something, or sit on the floor with a toddler, a structured suit will fight you the entire time. Stiffer fabrics don't forgive movement the way jersey does, and a fitted waistcoat has zero stretch to give when you've eaten more than planned at lunch. It's also not forgiving of a bad mood dressing day, you either feel sharp in it or you feel like you're wearing a costume, there's not much middle ground. Before you buy, check the lining, a fully lined blazer will hold its shape for years while an unlined one creases if you look at it, and check where the button sits when fastened, too high and it pulls across the chest, too low and the jacket gapes. Check the trouser break too, puddling fabric at the ankle makes even good tailoring look unfinished. Pocket flaps that gape open on the body rather than lying flat are a sign of a cheap cut, however nice the fabric feels on the hanger.
Avoid pairing sharp tailoring with anything equally fussy, a heavily structured bag or a stiff court shoe and the whole outfit stops looking like clothes and starts looking like armour. Soft knitwear underneath, a plain tee, something with a bit of give, lets the tailoring be the statement rather than competing with it.
Wool and wool blends soften with wear, the stiffness in the shoulders eases after a few months and it starts to move with you rather than against you. Cheaper polyester blends don't do this, they stay stiff and start to shine at the elbows and seat instead of ageing well. A waistcoat worn daily will show at the buttonholes first, so check those are properly reinforced before you buy. Dry clean tailoring rather than washing it, and hang it on a proper shaped hanger so the shoulders don't droop in storage.