In virtually all diving situations you'll need to add lead weight to offset your buoyancy in order to dive efficiently. The simpest way to do this is with a weight belt. Freedivers use a rubber (they aren't actually rubber anymore but the point is they stretch) strap belt with whats known as a Marseilles buckle.
Using a belt that has stretch makes an untold difference over a traditional webbing belt as it means that the weight stays where you put it and doesn't come loose as you dive and your suit is compressed.
The first thing most people say when they see a Marseilles buckle is "but don't you need to be able to ditch it quickly?" The answer to that is of course a resounding yes! but contrary to first impressions the Marseilles buckle releases the belt far quicker than traditional scuba style ones. This is because once the pin is released it is held open by the stretch in the rubber and the hole is so much bigger for the belt to slide through.
The major difference between the various options available is how much they stretch and how long they'll last. The belts we sell here don't have quite as much stretch as some others. We do this on purpose as NZ divers tend to wear quite a lot of lead due to the thick suits we use and the stretchier belts start to sag if over loaded. Because there are holes punched in the belt for the buckle pin it is inevitable that they will eventually tear and this is how weight belts eventually die. Top quality belts like the Rob Allen far, far outlast cheaper varieties.
Lead weights come in all sorts of shapes and sizes but the best for use with a freedive belt are buckles, usually available in 1 or 1.5kg sizes. The reason the buckles are best is it means that you are maximizing the amount of belt in contact with your suit which helps stop it from sliding around.
The million dollar question when it comes to dive weight is how much to wear. There is no universal answer to this as different body compositions will float more or less - fatter guys more, skinny guys less; people use different thickness wetsuits - more neoprene means more lead; and your needs change with depth - the deeper you dive the less you'll need. The idea is that you will be positively buoyant on the surface but negative enough at the depth you're working to comfortably lie on the bottom. This usually means you are neutrally buoyant at a third of your target depth. As a very arbitrary starting point you can use 10% of your body weight in a 5mm suit.
Aside from wearing your lead in a belt you can also wear it on a harness. It is important to note that you usually still use a belt as well and the harness just spreads it out a bit better. For divers who get a sore lower back they are a godsend. Also if you are using a lot of weight ie more than 10kg it is a real help to spread it out a bit. For the rest of us simpler is usually better and a belt is fine.