The perfect T shirt:
Short sleeved, crew neck, V neck, cotton, range of colours (seriously, all the colours), one fitted, one boxy
Long sleeved as above [see J Crew]
Well tailored trousers in a range of shapes and lengths in natural fabrics.
Knitwear as above.
IE, quality staples. When I want to replace all my long sleeved Ts, I would then only need to go to M&S. I buy many of my short sleeved Ts from J Crew (in US), as they've got a few different shapes, and tonnes of colours - so instead of their being 1 for me, there's about 5, so I buy more than 1!!
Oh and if your markerting strategy includes use of bloggers / Instagrammers, make sure you have LOADS of online stock of that item - people following them will go straight online, not to a store. I saw a blogger's dress, went to the link - sold out in every size. I don't particularly like going instore (and have very little time to anyway), as the stores are so confusing to navigate. Too much stock.
KISS - Keep It Simple, Stupid!