Instead of putting a paint smear across the outside of a can to mark the level of paint left inside (which can be messy and obliterate the label), use a rubber band instead. Put it around the can and roll it down to the level of the paint. Just by glancing at the rubber band, which is visible all the way around, you will know how much paint is left.
When you reorganize your basement or garage, buy yourself a number of cardboard boxes all the same size instead of scrounging various-sized used boxes. This way they will all have covers and you can stack them neatly. Mark each box with a big letter and number, like A-1, A-2, etc., on all four sides and top. Use a three-ring binder to write down the box number and its contents.