For acceleration, remember this:
acceleration = (final velocity - initial velocity) / time
Then as the player holds or doesn't hold the accelerate button increase or decrease the value by a small amount until you reach the maximum velocity.
When the car turns, make the current velocity go down a bit, because velocity is speed in a single direction, and you're changing directions.
For sliding, I'm not a real expert, but you could try keeping the car moving in the direction it was once facing for a while and slowly decrement it.
good luck,
Crazy Donuts
Juvenile Industries
Current Project: KillZone(FPS)
Soon to come:An rts, and a snowball fight game