First of all: the modules in the track editor appear smaller as they are in real game! Keep this in mind while layouting your track. You'll get a feeling for this after you exported some tracks.

I will not explain the general usage of straights and turns, but I'd like to explain some extra functions that will improve the overall layout of your track:

Straights
This tiles can be risen up to a 45° and it looks that most of the track builders around have found out that pretty soon! What I want to say: use elevation changes wisely and throw in some small changes here and there in order to keep the track flow natural and fluid. Do NOT make your track a hill climbing experience.

Rumble Strips
Mainly you should use this ones to slow down the gameplay. But it's not useful to connect 6 rumble strips just because it looks so funny when the cars start bouncing. Better place them in front of tricky corners or on long straights to interrupt a boring flow and make the track more of a challenge.

Dips
They can be well placed after a jump: you are forced to take the jump correctly in order not to get stuck in the ditch. A small dip (press 'Z' once) also works out as a slight slow down.

Humps
If you have risen up several straight tiles in order to make a huge dropoff, try to finish the jump with a hump (see screen 1). Also works out well as a 'fly in the curve': place it 2 or 3 tiles apart from a sharp curve (see screen 2) and you can try to jump into the curve by steering at the time of takeoff. A very cool thing!

Corners
Well, the usage of rounded corners is a question of taste: small ones slow down the gameplay and you have to steer hard. Big ones can be driven at full speed and don't stop the flow. Use them as necessary, your track won't do without them.

But I actually love square corners! It's a shame that they can't be enlarged, but even without it they're pure fun. Make sure to use them wisely, never throw more than 4 or 5 on one track! A special usage is shown in the screenshot below: square corners can be combined well with diagonals in order to make a straight track piece more interesting.

Chicanes
Bah, I don't like them! In single sized tracks it's not possible to got straight through them and so they become an annoying slowdown of gameplay. I only use them as a reward for a good jump (see screen): if you take the jump correct (what shouldn't be too hard) you don't have to drive through it.

Chicanes do work well in double sized tracks. They narrow the road which is far too big anyway.