Change of variables in 3 dimensions

(First post, hoorah!)

Hi there - I'm revising for my exams in a few weeks and decided to look at some of the older past papers for my uni course, but they tend to deviate off the syllabus a fair bit so I haven't been taught how to approach a problem like this systematically, I'm using mostly guesswork and logic...

(i) 'The domain S in the (x,y) plane is bounded by $\displaystyle y=x$, $\displaystyle y=ax (0 \leq a \leq 1)$ and $\displaystyle xy^2=1 (x,y \geq 0)$. Find a transformation

$\displaystyle u=f(x,y) v=g(x,y)$

such that S is transformed into a rectangle in the (u,v) plane.'

I figured I may as well aim to transform into the unit square if I could, since it's only a short step translating from a rectangle in the (u,v) plane to the unit square. The only problem which struck me here was the fact a rectangle needs 4 'bounds' whereas the given area in the (x,y) plane has only 3 bounds. Through, as I said, mostly fairly unconfident guesswork, I came up with $\displaystyle u=\frac{xy^2(y-ax)}{\frac{1}{\sqrt{x}}-ax}, v=\frac{xy^2(y-x)}{\frac{1}{\sqrt{x}}-x}$ - would this work? It gives u=1 and v=1 along the $\displaystyle xy^2=1$ boundary and 0 along the y=x and y=ax boundaries, so would that be okay? In general, what's a good method for approaching this sort of question systematically?

(ii) 'Evaluate $\displaystyle \int_D \frac{y^2z^2}{x} \, dx \, dy \, dz$, where D is the region bounded by

$\displaystyle y=x$, $\displaystyle y=zx$, $\displaystyle xy^2=1$ $\displaystyle (x,y \geq 0)$

and the planes $\displaystyle z=0$, $\displaystyle z=1$.'

Obviously a change of variables is the way they want you to go, and I'm happy with the actual process of changing variables, the Jacobian, the volume integral etc, but how do I actually work out -what- variables to change to? It's not like it's a symmetrical area which points towards spherical polars or anything like that, so I'm not sure how to proceed!

Thanks very much for the help!