Try to get well-seasoned hardwoods for that long slow burn. If you are buying your wood, know what to look for. If the wood is seasoned it will look dark, or gray when compared to green wood. If you split a piece of seasoned wood it will look white on the inside. It will have cracks running through each piece.
Unseasoned wood looks wet.
It is lighter on the ends where it has already dried out somewhat. The bark will be tightly attached to the logs. Buying green wood for immediate use is not a good value - even if it is less expensive than a cord of seasoned wood. You will lose heat as it burns off the moisture and run the risk of having your chimney coated with creosote. Always have your chimney cleaned at the beginning of the wood burning season.