As was said before, try to use what you have in the plane already first. Move around things like the battery and receiver to try and shift the CG of the plane. But when you do have to add weight there is no right or wrong way of doing it. I have used many different things to add weight to planes. Du-bro lead weights, coins, nuts and bolts, modelling clay with buckshot pressed in to it, or just about anything else that will fit into the area you need to add weight to. An important things to remember when you add weight to a plane is to make sure that what you add is secure and can't fall off or shift while the plane is in flight. One way that really works well is using Du-bro stick-on weights and secure them in place with epoxy, or by drilling through them and bolting them in place.

Hope this helps

Ken