When filling do not use boiling water, very hot water is fine but boiling water can cause splits or leaks and will shorten the life of the bottle.
Three quarters is the maximum, do not overfill. A hot water bottle will give out plenty of heat and work just as well when filled to the correct level. Expel excess air from the bottle by lowering it carefully onto a flat level surface until water appears at the opening.
It may help to use a towel to cover the hot water bottle when adding hot water, this will help avoid any burns. For the unsteady hand there are PVC hot water bottles witha much wider neck/filler.
When tightening the stopper make sure it is on correctly, finger tight should be adequate. If loose replace with a new stopper but always replace like for like, not all stoppers are generic.
Use the hot water bottle to warm up the bed or area and remove before the person gets in. This helps to avoid any accidental burns to vulnerable people or children.
This may sound like obvious advice but it does happen! Never sit or lie on top of a hot water bottle.
Always buy a hot water bottle that has the British Safety Standard BS1970 mark. There are low budget hot water bottles that fail to meet to the standard with many product recalls from cheap imports. Not worth taking a chance on a product that has to work safely each and every time. Latest British Standard update is BS 1970:2012.
If your old bottle is showing signs of wear and tear replace with a new one. Check for splits or perishing and make sure the stopper is a good fit. When storing drain the bottle completely, remove the stopper and store in a cool dark frost free place. Always make a thorough check when using for the first time. A good quality hot water bottle will last 3 years, budget bottles may only survive one winter season. Many of the budget hot water bottles suffer leakage problems with poorly sealed stoppers. It is worth replacing every 3 years with a good reliable quality hot water bottle, over its lifetime it will cost far less to replace than a budget bottle.