You've got to have your basics. When I find a basic that works, I wear it all the time! For me tanks are an essential, I wear them all year round. Layered, under sweaters, blazers, tucked into skirts and with shorts in the summer. These are my two must have, never live without tanks.
Long Tank - $5.95 - H&M
I wear this tank layered over a cami, I LOVE the long length and how its so lightweight.
Stretch Bra Cami - $19.50 - Express
I wear these camis all the time, under sweaters in the winter and on their own in summer. I literally live in these things!