lights, primary and backup If you use backup lights ... you can clip them to the chest d-rings and have them sit along the harness, secured at the bottom with a piece of inner tube or surgical tubing (or a piece of bungee cord tied around the should strap). A lot of people diving cold water and low viz use canister lights for a primary. The canister rides on the waist belt, and the light head sits on your hand. One of my regrets is buying gear was waiting so long to buy a can light. It is a delight to be free of heavy, handheld lights. While many attach their light as Lynne describes, I attach mine (inverted) to the bottom right of my BP.
knife A lot of knives are sold with sheaths that will slide onto 2" webbing. The knife can ride just to the left of your buckle, on your left-hand waist strap. I put my shears on my waist webbing on the right side, partially because I attach my can light to my backplate, rather than putting it on my waist band on the right.
reel I was taught to carry my reel on my left hip D-ring, although you can also clip it off to your butt d-ring. I attach my reel to my right hip D-ring.
smb You can clip this off to your butt d-ring, or put it in bungies along the bottom of your plate. I use bungees along the bottom of the plate.
lift bag Some plates will accept a storage pack for lift bags, where the bag rides behind your back. I carry a lift bag, in a sleeve, attached to the lower left of the plate.
signal mirror/whistle/etc. These are the kinds of small items that really do better in a pocket. And, westuit pockets are really nice to have (see below).
2nd reg (octo/pony) If you don't want to use a long hose/bungied backup system, you can clip your pony or octo reg off to one of your chest d-rings. If you go BP/W, go ahead and make the transition to a long hose primary and bungeed necklace octo.
