As the RNLI correctly remind us, a lifejacket is 'useless unless worn'. The same could be said for wearing a faulty lifejacket. The difference between the two is, perhaps, simply one of perception.
Life jackets, as opposed to rather more simplistic flotation devices, are well-designed specifically to both keep the wearer afloat, floating face up and in most cases, by way of an in-built harness, connected to the vessel.
Life jackets usually comprise an inflatable bladder packed within a hardwearing, rip-opening cover, closed by Velcro and/or a burstable zip. The jacket will have reflective tape on it and is inflated either by a co2 gas cylinder or, if that fails, a manual one way valve which you blow into. Jackets have a whistle, a water-activated light and a lifting strop. It should go without saying, but always wear crotch straps with a lifejacket, assuming that you want it to remain on you when you need it! Most jackets also have a luminous hood and face cover to help prevent drowning and early death from exposure.
Gas cylinders are replaceable, simply by screwing them into the mechanism. They can come undone, so it's worth checking that the bottle end remains intact and that the cylinder is firmly screwed in place.
Life Jackets can be auto-inflated, hydrostatically inflated or manually activated. Make sure the toggle that manually activates the piercing of the gas cylinder is visible and accessible when you're wearing the jacket.
Hydrostatic activation relies on submersion (about 10cm) before activating. Other auto inflate mechanisms usually work on the basis of a fast-dissolving barrier with a spring activated pin behind it. When the barrier dissolves, the pin springs into the cylinder and inflates the jacket. It's important to check the firing mechanism regularly and most owners will have their jackets serviced once every 12-24 months.
When checking the firing mechanism and cylinder it's worth looking for corrosion too. Inspect the whole jacket, including the bladder, and make sure there are no leaks or worn areas. Check the strops for wear. Test the light and make sure it works. Batteries can quickly run out when a light is accidentally activated.
When re-packing a lifejacket ensure that you use correct technique and be careful not to snag the inflatable bladder when zipping up the cover. Make sure that toggle is not zipped inside the cover!
If your boat is coded for commercial use, your life-jackets should be checked and serviced at least annually. It's also prudent to carry re-arming kits when on passage so that jackets can be quickly recommissioned.
When checking your life jacket it's worth also checking your harness lines for wear. Check the overload indication material is still intact and that the webbing remains unworn or damaged by UV. Both clips should be working easily.
Finally, when managing lifejackets, it's sensible to keep a log of service intervals and keep any service certificates on board.