The above advices are valid for a normal storm when winds are up to 10 knots in strength. For anything above that your strategy must and will change. But in any case the best advice is to use common sense and if in doubt - don't go out.
1. Check the weather forecast. You need to know what weather you can expect before you leave the marina and during your time at the sea. Although this is obvious and common sense thinking, still, it is incredible how many ships leave port without that particular information. You can get the latest weather forecast information in the marinas, on the Internet, over the VHF stations etc ...
2. Never leave port/marina if the weather forecast is unfavorable. Don't think that bad weather will eventually pass or you will out run it. Plan and make arrangements for all the crew or guests to be ready for all weather conditions. If they are not ready they are not eligible for the trip.
3. When out at sea, continue checking and looking for signs. Every hour, check the sky and the barometer. Safe and correct way is that one of the crew has strict obligation to record and the weather into a ships log. Having such procedure will insure that you are not caught off guard.
4. Trim the sails before the weather gets worse. As soon as you notice the storm is picking up, secure the loose items and equipment on deck in the cabin, make sure all windows and openings are tightly closed, and close the drain valves for water (and feeder for sea water, if any). Also check all the ropes so that you don't need to do it during the storm and to avoid complete chaos on board.
5. Make sure that you trim the sails through the night. Unless you sail through a very quiet and foreseeable weather trimming the sails before the night is a good idea. You really don't need the panic of sleepy crew trying to trim the sails at low visibility at night. Trim sails to be sufficient for the weather, remember you are not racing so you don't need the maximum from your boat. All you need is to get through the storm safely.
6. Keep all the valves for sea water closed to ensure water tightness of the boat. With the bad weather is good all the time to have all these valves closed, except those that are needed at the time.
7. Do not seek for shelter. If you are already in bad weather, go to slightly sheltered are of the open sea, in the deepest water you can find. If your boat is prepared and in good conditions this is much safer option than taking shelter in the shallower water. If you trying to navigate to the port or marina during the storm there is a big possibility that you may run aground and hit something, so stay out in deeper water.
8. Wear appropriate clothing to ensure that the body temperature is stable and ensure the same for everyone on board. Hydrate yourself with hot beverages.
9. Worry about own safety. Stand at the safe place and be bound. Also, and all the crew should be bound for the boats railings and make sure that everyone wears a safety vest.
10. Never go to the bow without someone remaining in the cockpit.