Turbulence can have a variety of causes. Clear air turbulence (CAT) is associated with jet streams at altitudes between 23,000 and 39,000 feet. If there is a change in wind strength of more than about 6 knots per 1000 feet of altitude change, then moderate or stronger clear air turbulence is probable. Strong winds over rough terrain such as mountains and fast moving cold fronts are also a source of turbulence. Rising columns of hot air called thermals will also cause a bumpy ride. When enough moisture is present, cumulous clouds will form as parcels of air cool at higher altitudes. 

PIREPS are the best source of information on the location and intensity of turbulence.

Although turbulence may be uncomfortable, it can usually be navigated safely by following a few simple procedures. First, reduce power and slow the aircraft to the recommended turbulence penetration speed as described in the POH/AFM. Gusts can cause high wing loadings and increase or decrease the angle of attack, possibly causing the wing's critical angle of attack to be exceeded, resulting in a stall. To minimize the load factor imposed on the aircraft, the wings should be kept level and the aircraft's pitch attitude should be held constant, while allowing the aircraft to fluctuate up and down, since maneuvering to maintain a constant altitude only increases the stress on the aircraft (If necessary, request a block altitude clearance). Keep power at a constant setting that will maintain the recommended turbulence penetration airspeed.