1. The Garage - your own personal skateparkIf you have a garage with a concrete floor, then you're all set. Get the cars out of the way, move all the boxes into the attic, and you have a perfect mini-skatepark just for you. If your garage is big enough, you can even put in a miniramp to skate on, or a short rail to practice grinds. Garages are perfect for skating when it's wet and cold outside. Plus, if you are a new skater, no one can see how bad you are!
2. Carpet SkatingIf you have a carpeted floor in your house, like the living room, you can practice things like your kickturns, ollies, kickflips, pop shuvits, manual balance practice... really, all the technical tricks you want! Just clear enough space so you don't take out a lamp or anything.
3. Indoor Training EquipmentThere are companies out there who make special training equipment for skateboarding. For example, the Indo Board is a balance trainer. I have one of these set up in my living room pretty much all the time, and hop on it when I'm on the phone, or watching TV but don't feel like sitting down. Improving your balance over the winter will help your skating in the spring.
There are also other practice tools, like Ollieblocks or Softrucks. Both of these do basically the same thing - they help you practice your ollies without rolling, and without having to be outside (read Ollieblocks Vs Softrucks for a comparison). Since the ollie is the basis of most street technical skateboarding tricks, Softrucks and Ollieblocks are a great idea for practicing skateboarding throughout the cold, wet winter months.
4. Indoor SkateparksThis might seem obvious - in fact, I almost didn't include it in the list, but sometimes we don't think of what's right in front of us! If you have a local indoor skatepark, then go there! It might cost some money, but usually it's not that much, and if you go a lot most places offer a membership. If you aren't sure if you have an indoor park nearby, then call up a couple of local skate shops and ask whoever answers the phone. They should know.
If you don't have an indoor skatepark, you can always push for getting one built! Lots of youth organizations, churches, city parks and recreation offices, groups like Campus Life and others devote themselves to providing safe places for young people to be active. Check with organizations like these, and volunteer to help get something started!
5. Skateboard VideosWatching skateboarding videos does several things - first off, it's just fun. Second, you get all excited about skateboarding, and want to go (which might be frustrating over the winter). But third, you also learn stuff. Watch the pros and see how they do their tricks. Watch your favorite skater in slow motion. Study them. They may look larger than life to you, but to be honest, you could do the same things if you decide to. It just takes time, practice, patience, and will! Check out some Skateboarding Video Reviews for some skate video ideas.
6. Skateboarding Video GamesIf all else fails, you can sit on your backside and play a skateboarding video game. This isn't the best option at all, but it might give you that fix, and it should help keep you amped up over the winter for skateboarding in the spring! Take a look at the Skateboarding Games Category for more on skateboarding games.
7. Snowskates and SnowdecksIf the ground is completely covered in snow, you can always take advantage of it instead of avoiding it. Snowskates are like mini snowboards, without bindings. Another way to look at them is like skateboards without trucks. Snowdecks are similar, but have trucks with a mini-snowboard bolted to the bottom. Snowdecks and snowskates are a fairly new idea still, and these names often end up getting swapped around, so don't be surprised if you see or hear one being called the other!
Both snowdecks and snowskates are designed to let you have the fun of skateboarding when the ground is covered in snow. A lot of big ski and snowboarding lodges have even created terrain parks just for snowskates and snowdecks! They aren't the same as skateboarding, but they are close in a many ways, and fun to ride. See some Reviews of Snowskates and Snowdecks for more ideas.
8. SnowboardingA lot of the skills from snowboarding translate over to skateboarding. Most don't. But, it's riding a board, and it keeps you active! The drawback is snowboarding costs a lot of money ... Check in on the About Snowboarding site for more snowboarding info, help and ideas.