Many boats have cleats and chocks that are woefully inadequate. This problem becomes critical when more and larger-diameter storm lines are used during a storm. If necessary, add more and larger cleats and chocks now; they'll make securing the boat easier all year.
Assess the ability of cleats to carry heavy loads. This means making sure all are backed properly with stainless steel or aluminum plates. On sailboats, winches (if backed properly and designed for the job) and even keel-stepped masts can also be used to secure lines at a dock. Note that anchor lines should not be secured to the mast, as it creates that much more stretch on the line at the chock, which further increases the chances of chafe failure.
Don't put too many eggs in one basket by leading numerous lines to a single cleat, even if it's backed properly. Two lines per cleat is usually the maximum. Also, a cleat is not reliable when lines are led perpendicular to the base and the cleat can be wrenched out by the tremendous loads (see diagram below).
Strip all loose gear that creates windage: canvas covers, bimini tops, outriggers, antennas, anchors, running rigging, booms, life rings, dinghies, portable davits, and so on. Anything on deck that can't be taken off should be lashed securely.
Unstepping masts on sailboats is strongly advised. If this is impractical, sails (particularly roller-furling headsails) must be removed. Roller-furling headsails create a lot of windage, especially when they come unfurled, which is almost guaranteed to happen no matter how carefully they're secured. All halyards should be run to the masthead and secured with a single line led to the rail. This reduces windage and minimizes flogging damage to the mast. The line can be used to retrieve the halyards.
Electronics and other valuable gear should be taken home for safekeeping. Not only are electronics vulnerable when vandals comb through boatyards after the storm, they can also be wrecked by all of the water. Personal belongings and other loose gear should be taken home and the cabinets and cabin doors secured. All ship's documents should be taken off the boat.
Preventing Water Damage
Remove cowl ventilators and seal the openings. Use duct tape to cover instrument gauges. Duct tape should also be used around hatches, ports, lockers, and so on, to prevent water damage below. Note that some types of duct tape leave less gummy residue than others.
Close all but the cockpit drain seacocks and shove a plug into the engine's exhaust ports. If the boat does take on water, it will sit lower, and water could back up into the engine. Remember to remove the plug before starting the engine when the storm has passed.