Don't overdo it, as it could look too studied. A linen suit will achieve the affect naturally and without artifice. Or simply sleep in your clothes for a night. All these pictures are from the Sartorialist.
Rumpled, dishevelled, untidy, scruffy, tousled, mussed up, messed up, messy, disturbed, ruffled, jumbled, topsy turvy, disarranged, discomposed, disrupted, disorganised, disordered, disarrayed, broken up, in a shambles, slapdash, slipshod, cluttered, chaotic, confused, sloppy, slovenly, unkempt, unfastidious, bedraggled, ungroomed, unpolished, grungy, rough, creased, neglected, shabby, bedraggled, tacky, etc etc.