well, studies show that internal motivation - the pleasure derived from the work itself, from the process rather than its finality - is the best kind. however, this is built in childhood, through education, and not everybody has an inbuilt motivation system.
so for most people, a very good motivator is having a great workplace with kind and competent colleagues and a fair rewarding system (money, incentives, medical insurance, discounts for various types of entertainment). Also, a reasonable timeframe (no more than 40 hrs /week). sounds pretty simple, right?