Hi and Welcome, magicm!
Great Topic!
First, I look for the real existing model of what I want to build-mainly construction equipment.
Second I scale it down using PDF provided by manufacturer. I try to build everything perfect in scale, usually 1/13 or 1/20.
Third in my mind I calculate few days to see is everything possible, even in theory.
Fourth I use MlCad to create virtual models. That is because it is easier for me to see completed model in advance, and to see is it really possible create what I want.
Fifth if I lack of some parts (usually I do) I try to find them all in ONE Bricklink store, to cut down shipping price.
Sixth I enjoy in building the model from MlCad from my computer. That is very hard and long process because CAD only shows rough model, I have to work on numerous mechanisms while I am building. I like that very much and enjoy doing that.
My advice to you if you want to build exclusively MOCs, is that do not buy sets, but only Bricklink parts which you will need. That is way cheaper, but more dangerous, you can order parts and almost regularly find out that you miss (very) few important pieces. So (very long) planning is essential, for me.
Hope it helped a bit...
Happy building!