Warm up before you start work by doing some gentle neck, back, and shoulder stretches. Sit with good upright posture while you drive. Keep your chest up and shoulders back. Try to avoid straining your neck when you turn to see behind you. As much as possible, turn your shoulders with your head, so that you aren't just craning your neck. Assess whether rearview mirrors on the forklift could help you see behind you without having to turn your head as often.
Take frequent breaks as you work. Do some gentle stretching or walk around a bit. During your breaks or after work, try using a cold pack on the sore area. This can help control the pain.
If your neck pain continues, see a doctor. Your doctor can help control the pain, and he may be able to help you come up with exercises to help strengthen your neck muscles and improve your posture.