A properly weighted and trimmed scuba diver experiences less drag and easier buoyancy control all while reducing the amount of energy they expend. Ultimately this diver will have more fun and create less environmental damage. Despite its importance, effective weighting remains a mystery to most divers. Appropriate adjustment of weight (amount & placement) is probably the single most important component toward enabling divers to minimize their effort while maximizing the amount of fun they have on a given dive.
Diving with a balanced equipment configuration is essential to the safety and efficiency of any diver. On one hand a diver should carry enough weight to be able to hold a shallow stop at the end of the dive with little or no gas in the wing. At the same time, the diver must not be over weighted at the beginning of the dive with full cylinders; they should be able to swim from the bottom even with a failed BC.
To check for a balanced configuration, try to hold a 10ft/3m stop with nearly empty cylinders (approximately 500psi/30bar), adjusting your weight accordingly to be neutral with an empty BC. With this information, analyze the amount of detachable vs non-detachable weight so that you are able to comfortably swim up the same configuration with an empty BC and full tanks.
Now that you know how much weight you need to be detachable, you can make adjustments for trim and decide how to place the weight in your configuration. Remember, if your configuration required any detachable weight, you must keep this removable.
Congratulations! You are now properly weighted. Your configuration will vary from time to time and as such, you should recheck periodically or when you are making an environmental or equipment change.