I use SolidWorks for my CAD stuff, it's taken a long time (over 2 years) to get to where I am with it - and I'm still doing very basic stuff really, that a proper CAD designer would do in his sleep. I find the best way to make up a 3d object is to use lots of simple 2D extrusions/cuts; Solidworks (and presumably other CAD software also includes some really quite natty tools to get where you want to be; the temptation to use them can be strong (e.g. 3D-sketches), but I find if you can possibly avoid them, life is much easier.
It took me a little over 1/2 day to draw my V12 valley plate c/w dizzy drive from scratch (making quite a few revisions along the way); I had to learn a few new techniques to make it work for me (e.g. drawing on an angled plane, to get the dizzy drive angle correct), but the end result is great.
Visualising the part you're after is half the battle, the rest is just messing around with lines & relationships (coincident, tangent, colinear, etc. etc.)