If you are stuck with keyboard, the advice to tap, tap, tap to steer is the best one. The easiest hardware solution is to get a cheap, wired XBox 360 controller. They plug in through standard USB, Windows natively supports them, as does the game.
I was using keyboard/mouse/Logitech G13 Gaming keyboard/controller for everything but driving and then switching over to the 360 controller for driving. Made things a lot easier, as the 360 controller provides analog input, (the wheel position is determined by the precise position of the controller thumb stick).
In my case, after a little searching I found a utility program that allows the G13's thumbstick to simulate the analogue output of a 360 controller. (The G13 thumbstick is analog, but a game needs to be programmed to accept analog input from the controller and few games offer that support). I still find the xbox controller to be a little more precise, but I like not having to switch to the xbox controller anymore.
(It would be really nice if Logitech would offer xbox stick emulation natively, but I assume they might have to pay some sort of royalty to MS)?
Anyway, yeah, an analog thumbstick definitely does wonders to vehicle control in the game, so it's worth exploring.
