Maybe not a beginning question, but what do you do with the boxes, instructions, extra parts. unused decals and sprues once you finish a project? Pitch, sort, save?

I keep the instructions, decals and many parts. I tend to throw out the boxes because if I didn't I wouldn't have any room for new kits

Same here,get rid of the boxes,keep the rest for spares,not everything though,decide what will be useful and what will not,otherwise the spare parts can get out of control too.

I keep spare parts and decals in ziplock bags... the boxes I usually use for spray painting/airbrushing during the building process if they are top-opening, though lately I have taken to cutting out the box covers and pinning them to the wall around my workspace.

I save everything. I cut out and keep the top of the box. I save the instructions. Extra parts/decals go into the spares bin. I even cut up some of the spruce to keep nice long straight spruce runs (good for melting and making thin wire/lines for antennas or even rigging). Hoarding is a positive trait in this hobby.

Cheers

I keep the artwork top of the empty box in a file. The rest is recycled. Its nice to look at the artworks every now and then to see how many and what I've built.
All spare parts are kept in a seperate box. Its a waste of time, though. I've never had to use any of the left-overs.