Multiple screens is another way to do it, with two PCI-E video cards (second one doesn't have to be up to much) and three 1280x1024 screens you can see all your mirrors at once.

Software you need is Kegetys's SoftTH (google it). Not the cheapest way, but it does give you a good field of vision...

I usually mount a camera on the centre of the dashboard looking over the bonnet, or when in a formula car on the T-bar section. I use a FOV of around 60 degrees.

I really dont need to see where the cars are either side of me to know where they are, often other cars are exactly where I intended for them to go, as part of race craft isnt just anticipating what your opponents are going to do, but manipulating it too.

Of course there are times when a driver may do one of two things, and for these ocassions I have look left and right bound to my wheel. You can look behind by pressing both together, although I preffer to use the virtual mirror for that.

When you are starting out you might find Pit Spotter useful. Getting used to the virtual environment and the differences in the human/race interface can take a while. I used Pit Spotter a lot until I was used to the game and felt ready to drive on instinct and I strongly recommend the program for all beginners who want to cultivate their skill at sim racing. You can find Pit Spotter in the mods section of the forum somewhere.

As I very rarely drive the single-seaters, I have the gearshift paddles on my wheel mapped to look left/right, making them nice and easygrab in the heat of a race without having to think about it too much.