OK I'll weigh in.
5'11" and 150lbs and I have some comments about this, while I'm certainly not as slim as many who are consulting on this post I do feel that you guys are making some comments that dont necessarily cover the whole gamut.
Khakis: these are certainly some of the toughest items to look good in, but they can be done right.
The key: No break, infact i like to leave them long and roll them up to the point where they dont touch my shoe.
Tapered, they have to be tapered below the knee.
Dont taper the full leg too much.
To keep cool in the summer consider linen pants instead.
Dont take the advice that baggy sweaters look good, they dont. Buy a size that fits snug, with appropriately sized Neck openings, collars, ect.
Aside from fashion brands like Dior, RLBL, ect. I feel that in suiting and sportcoats the look that works is a soft shoulder, slim through the arm, a hair full in the chest, and slim at the waist.
Single vents work fine for me.
Buy the right neck size, i went up until recent with 1/2" too big in the neck, its the size i started with and i just never sized down because i never went through the effort of trying on shirts of different sizes in the store.
I've always gotten my shirts tailored, its part of looking sharp.
Dart the shirts! but dont go crazy, too tight looks bad, just as bad as too loose.