Now's the best time to inspect your chimney.

Chimney flues, whether metal or clay lined or made from brick or stone, must be properly maintained to prevent smoke damage, carbon monoxide poisoning or even a house fire. The interior of the chimney should be inspected to make sure it is unblocked and free of excessive creosote buildup. The exterior should be inspected for cracks and inadequate flashing. Check that the damper is open before starting a fire.

During periods of disuse, it isn't uncommon for creatures to set up a home in a chimney. Birds, squirrels, raccoons, bees and wasps are the most common critters found in chimneys and their nests can block air flow from a fireplace. This will make it more difficult to start a fire and more importantly, it may cause smoke to flow into your home instead of up the chimney.

A chimney cap will help to keep creatures out. Use of a spark arresting chimney cap (recommended) will also reduce the risk of a roof fire from floating embers.
Creosote buildup is another important thing to inspect for. Burning wood, especially green wood, results in the accumulation of creosote, a tar-like substance, inside the chimney. Creosote is flammable, and an excess build up can lead to a chimney fire. A build-up of 1/8 inch or more increases the risk of a chimney fire and should be removed.
Inspection of the chimney is best done from the bottom and the top. If you aren't comfortable climbing up on your roof to look down your chimney, you should consider hiring a professional. If after inspecting your chimney, you find that it requires service, this is work best left to a professional.
Use a mirror or lay down newspaper to look up the chimney. Use a powerful flashlight to light your view. When inspecting, look for blockages, cracks, damage and creosote build-up.
Inspect the mortar joints in brick or stone chimneys. Gaps can allow water in, which can lead to extensive damage and it can allow carbon monoxide to flow into a room in portions adjacent to living areas.
If you discover a nest in the chimney, you can try to scare away creatures by wadding up a single piece of paper and burn it in the fireplace. The smoke will be minimal but may be enough drive out any lodgers. You may be able to dislodge a blockage using a piece of lightweight PVC pipe. Be sure to use a long enough piece that it won't become lost in the chimney if you drop it. Only attempt to clear a blockage if you can do so safely and have a stable position to work from. Creosote clean-up should only be done by a professional.