If you have the space and the cash, I'd suggest looking into rollers. TruTrainer rollers have a built-in flywheel resistance mechanism that effectively mimics the wind resistance you get by riding outside; other brands, such as Kreitler, offer resistance units as an add-on. I have TruTrainer rollers and am very happy with them, though in hindsight I wish I had bought the model that lets you disconnect the resistance.

I recommend rollers rather than a trainer for two reasons: the resistance is proportional to speed (at least with TruTrainer), so you can change resistance just by shifting gears, and riding rollers is less boring than a trainer, because you have to pay attention so as not to fall off. They really do teach you to hold your line.

Try before buying, though, if you decide to go with rollers. Some people find them hard to master.