Making sure to uncheck merge bodies on each extrude you do, you can easily keep working off of solid objects, using the sides of objects as sketch planes, and surfaces to extrude up to. If you then use the split part tool, you can save each individual part with its own file name! and since all parts are created in context of the origin, in assembly you drop each part onto the origin and there are no interferences, and everything is fully defined in space without any relations. This has saved me many many hours... thought I would share, it seems obvious now, but some text books dont cover this at all.

If you copy a sketch, just pick the plane you want it pasted on and ctrl V. Do not select sketch first, just pick the plane and paste, this drops the sketch in with the same relation to the origin as the original.. You then have to pick the sketch and rt click edit sketch. But if you do a super detailed rt view of a complex object you can keep pasting it in exact relation to the origin , convert the whole sketch to construction geometry and then just convert back the lines you need and extrude, cut, or revolve as necessasry., short of converting splines, I have trouble moving sketches around if I paste them randomly. Work smarter not harder...