Let's forget about the whole debate over whether or not the sandbox exists. Only a few people would say it doesn't, so let's move on and talk about what to do if your site is stuck in the box. There are several ways to deal with the situation, and which ones you choose depends upon your individual situation.
First, there is the DaveN redirect solution which involves duping the content on an old domain and using a 301 redirect to the new site. I'm not sure this will work in all situations, but it may be worth a try.
Second, you can ignore the sandbox completely, and simply market the hell out of it, as Mike advocates. It may or may not actually help your rankings, but you'll get traffic from lots of other places, so you may not have to care about the sandbox. However, this will likely cost you a LOT of money, so this may not be feasible for everyone.
Third, is to find a way to generate lots of buzz around your site. There are many ways to accomplish this, but some include being controversial or funny, or by offering something new and worthy of acquiring lots of natural links, such as an awesome new free tool. This can be an effective way of busting right through the sandbox, but it usually requires some creativity. You'll need to think about how best to get the users (often bloggers) excited about whatever you have to offer, or whatever you have to say.
Fourth, and this is my preferred choice in many situations, is to target the long tail. The sandbox is far less likely to affect long tail terms. If you can bring in the users via hundreds or even thousands of less competitive, niche terms, then the fact that you don't rank for one or two big terms may not matter much. While you are catering to the long tail searchers, you are earning money (hopefully). By the time the sandbox period ends, you have already established a useful, revenue-generating site that will stand a good chance at pleasing the Google algo gods.
So let's move on and find ways to get users to your site. If you have original ideas for doing so, you could beat the sandbox, without having to ever debate whether or not the sandbox actually exists. 🙂