MUST HAVES  |  PANTS & TROUSERS  |  Khakis/Chinos

or construction workers and cowboys. The "go-to" trousers for preps are Khakis (US - name derived from the color) or Chinos (UK & Ireland - also the name of a rugger-bugger pub game).
These are light colored, relaxed-fit pants that pretty much go with anything. These are primarily for casual use so I would caution against wearing in the office.

How you wear these pants says a lot about you. Personally I try to avoid pleated chinos and never wear chinos with a crease down the front of the leg as this is too stuffy and structured a look. 

These look great with a polo shirt - tucked in, natch. Complete the look with a needlepoint belt and a pair of well worn boat shoes! Bills Khakis are the best (as shown above), but Tommy Hilfiger, Charleston Khakis, Jack Donnelly and Dockers all make excellent alternatives at reasonable prices.