@ Kiflom: Thanks too
I'm not too sure what would be my workflow... It is a bit different in every project. I think maybe a bit of a bazaar-style, unix-like agglomeration of small parts. I usually don't do anything during several weeks, but the project is always in a corner of my mind. So, these weeks, even if they are not very productive, have their utility. When I start sketching then, ideas usually come easily and partly organized already.
Then I sketch a lot, I would say all the concept construction and testing is done with hand sketching. When I get a firm enough idea, I put some rough environment in blender, like linework, imported 2d plan, etc... and I start to model my sketches in 3d. Usually at this point you see if the idea is nice or not.
Of course your own inconscient is always very reluctant to throw an idea away, at least mine. So I must keep very vigilant, and if I detect that I'm not totally happy, I usually must force myself to throw away part of the idea, or even the whole idea. Then, go back, etc...
For a competition, I usually don't go much further than that. I refine my model more and more, and when I think it is "solved" enough, I decide how I will show it on the panels, what technique I want to use, etc. Then, if I will need plans and sections, I usually extract them form blender and refine them in another program like inkscape, gimp or even autocad sometimes.
The final design of the panels, I usually do a mix of vector and bitmap, so I use both inkscape and gimp.
Hope it helps!