Buy a whizz wheel, draw it on paper using lots of examples and check using the whizz wheel. After about thirty it becomes second nature and if you can do it, you are a shoo in for your nav exam.

Triangle of Velocities - ATC link for Info is BROKEN
 

Sept 2019:
Just tried to follow this link.
BT responded it no longer exists

What do you expect after 6 years? The Wind has shifted.

Buy a Jeppessen CR type and avoid the slide carry on. Easy to use, available in a range of sizes - 2 of which will fit in a pocket, and all of them will take you to ATPL exams if that is your goal. Even better, some of the ATPL problems require fewer steps compared to using Pooley's slide type.

Jepp don't sell their smallest one anymore, but APR Industries have their own range of them.

Conor Sproat

The Bad News: You have to learn how to calculate the wind triangle in order to pass the theoretical exams. My advice is don't overthink it just practice the mechanics of solving the problem on the wizz wheel. The part you care about is having a common sense understanding of the validity of the answer the wizz wheel gives you. (e.g.So if the wind is from the right of your track your heading to fly should be more than you track. If the wind is ahead of you then your groundspeed should be less than your TAS)

The Good News: In low and slow airplanes all that figuring is not actually required to get from A to B since what you end up doing is adjusting the heading to what ever it takes to follow your track by means of what you see looking at ground features. You still need to know the general direction and speed of the wind but the absolute value you calculated probably won't actually work perfectly because low level winds can be quite variable so you are still going to be adjusting as required to stay on track.

You will probably never use a flight computer after passing your exams, so just get a cardboard or plastic one. The slide type is good for checking your answers, since you can visualize the wind triangle.