One of the things I hate about playing games like 40k is flipping through rulebooks looking for information on units. I always liked the unit cards that came with the warmachine models so I decided to do something similar for my 40k force.
I put together a quick template and added pictures of my units I took on one side. I also added a short background blurb. On the other side is the statblock and a short description of pertinent rules. I print the cards out at index card size and stick them into photo sleeves. That way I can write on them with a dry erase marker, put extras into the pockets like psychic power cards or whatever else I need.