There is an absolute difference between looking stylishly nonchalant (the lived in-look) and just plain sloppy.  The difference is this: wrinkled shirts are sloppy and tactfully rumpled shirts aren't.  The laid-back season is the ideal time for laid-back clothing - stray too far from summer and your bound to radiate "bad hygiene" to those around you.

There are two ways to achieve the lived-in look: Do it yourself -or- buy clothing that has already been broken.  To truly personalize the look, the DIY method is highly recommended.

Do-it-yourself lived-in look:
1) Begin by placing your favorite stone-wash cotton items in the washer with one half cup of rock salt.
2) Wash them on the heaviest setting
3) Air dry
4) Neglect your iron

A truly rumpled shirt or jacket becomes immediately yours when you put it on.  Be sure not to wear more than one rumpled item at a time or else you risk looking as if you had just woken up.