This is probably common knowledge. Use labels. Lots of them. Label the crap out of your crap.
Here's my tip though - don't spend an hour on your computer printing out "pretty" labels or going on Pinterest to see how some made up person labels their attic crap. Just grab a sheet of labels (if you have labels laying around like I do) and a Sharpie. Just write what you're storing. My handwriting is horrible - but I can read what I wrote and it took me about 5 seconds to create each label. I have a bunch of Rubbermaid bins - so I put a label on the side and top. Maybe redundant, but easy for me. Also - if you use labels, they come off really easily if you ever switch the containers contents.