A proper warm-up consists of exercises to increase blood circulation in the golfing muscles, plus stretches for these muscles. Pre-game stretching reduces the chance of injury and improves performance (pre-stretched muscles can exert more force than non-stretched muscles). What pre-game stretching won't do is increase body temperature. Since cold muscles and tendons are more prone to injury than properly warmed-up ones, stretching should follow 5 minutes of light exercise.

Exercises that increase blood circulation in the golfing muscles:

Walk around or in place

Step Sideways
Right foot out and back, left foot out and back.

Arm Swings
Keep arms straight as you slowly cross arms in front, spreading shoulder blades apart, then slowly swing arms out to the side while squeezing shoulder blades together.

Arm Circles
Keep arms straight as you slowly circle left arm up, back, then down. Repeat on right arm.

Partial Squats
Stand with feet hip-width apart and hands on hips. Bend from the hips and slowly sit back 2-10 inches. Make sure your knees don't go past your toes!

Stretches that increase flexibility in the golfing muscles:

To stretch properly, go slowly and gently to the point of mild tension. Hold each stretch for 10-30 seconds--no bouncing! Perform a stretch for the same length of time on each arm or leg. To relieve sore or tight muscles, stretch after your game as well as before.

Low Back
Sit with good posture on a bench or golf cart seat. Slowly turn to your right to a point of mild tension. If it's comfortable to do so, grab the back of your seat. Keep feet on floor. Hold, then repeat on left side.

Hip
Sit and cross right leg over left with right ankle resting on left knee. Keep back straight as you lean forward to a comfortable position. Hold, then repeat on left hip.

Hamstring
Extend left leg with knee straight on a bench or golf cart seat. Keep back straight and chin up as you slowly reach toward toes. Hold, then repeat on right leg.

Shoulders and Arms
Grip head of golf club with right hand and extend it behind your head, letting the club hang vertically. Reach up with the left hand and grab the club as far up the shaft as you comfortably can. Gently pull down with your left hand until you feel a mild stretch in your right shoulder. Hold, then repeat on left shoulder.

Chest and Shoulders
Standing with good posture, hold a golf club horizontally behind your back with both hands, palms facing out. Slowly raise your arms until you feel a mild stretch in your chest and shoulders. Keep upright--don't lean forward.

Side Bend
Standing with good posture and feet shoulder-width apart, hold club horizontally above your head with both hands. Slowly lean to the right until you feel a mild stretch along the left side of your trunk. Hold, then repeat on opposite side.
For prolonged improvement in flexibility, you should stretch after your strength-training workout or other exercise, when your connective tissue and muscles are thoroughly warmed up. Another good time to stretch is after a hot bath or shower.