How to prepare your plants for winter depends greatly on the type of plants that you have and what the winter weather is like in the region in which you live. You obviously wouldn't winterize carnations in California in the same manner you would winterize roses in Rhode Island. But, there are still some general steps that any gardener can take to make sure that plants stay healthy over those long, gray winter months.
Move plants into sunny areas during the day. This generally applies more to indoor plants but also can be applicable to outdoor potted plants as well. With the reduction in daylight over the winter, it is important to optimize the amount of time your plants have access to full sunlight. This will likely mean moving your plants around a few times during the day to make sure they are getting as much sunlight as possible.
Lay down a thick layer of mulch and compost. For those outdoor plants, provide a nice layer of compost and then a thick layer of mulch covering. The compost will help provide food for the plants over the winter, and the mulch will add a layer of insulation to help keep them warm.
Move plants in stoneware pots indoors. While stoneware pots can be lovely, they do nothing in terms of providing insulation for your plants come winter. If the temperatures fall below freezing, that stoneware will actually catch and keep the cold, almost guaranteeing that your plants inside will die. So, if a freeze is coming, move those stoneware pots inside to somewhere warm.
Cut flowers back to 2 or 3 inches above the ground. The longer stem sections and exposed bulbs of flowers are the most susceptible to the cold. By trimming these portions back to just above the ground, you reduce the exposured area and protect the roots. If you live in an area that gets particularly cold, you can also cover the exposed portion of the flower with burlap sackcloth or plastic sheeting to provide additional insulation.
Encourage potted plants to go dormant. You can move your potted plants to a cool, dark area that will remain between 40 and 50 degrees throughout the winter. They will soon go dormant and can be left that way until the weather becomes sunny once more.