If you arrive at your final destination in the morning, you'll want to get off the plane as rested as possible. Otherwise you might head straight to bed while the rest of the world goes about its business.
However, loads of sleep during the flight can actually be counter-productive if your flight arrives in the evening: you'll be so rested that you can't fall asleep--until the following morning. In this case, it's better to arrive tired enough that you'll sleep no matter WHAT time it is.
Board the plane armed with the following: an eye-shade to block out light; ear-plugs to shut out sound; and an inflatable neck-rest for comfort. In addition, pack a high-carbohydrate snack such as fruit, candy or bread products--it's more likely to induce drowsiness than one that's high in protein. Finally, wear loose clothing and dress in layers. If it gets too warm, you'll be able to shed a layer. Also, bring a warm sweater or sweatshirt just in case it gets chilly.
You may want to consider taking melatonin, a hormone that some people find to be an effective sleep aid over the short term. It's widely available in drug stores, health food stores and now even grocery stores. A prescription isn't necessary, but as with any pharmaceutical, you should consider consulting a physician before trying it.
You might also consider taking a sleeping pill, but you should always consult with a doctor before doing so. Sleeping pills can be risky, especially when taken at high altitudes. Avoid taking either sleeping pills or melatonin for more than short periods of time.
Planes seem designed to prevent sleep, but you still may need some extra help. Bring along a stimulating activity, whether it's a mystery novel, your taxes, or a pack of cards. When you start to dose, get up and walk around the cabin, splash some water on your face, or engage willing neighbors in conversation. Pack a high-protein snack, for example unsalted nuts or boiled eggs, and drink some caffeine (coffee or soft drinks are typical choices) if necessary--though whether or not you want to consume caffeine depends on your own diet. Needless to say, avoid drinking caffeine as you approach your destination, or you may be too stimulated to fall asleep once you arrive.