Understand your battery
Most mobile phone batteries these days are lithium-based rather than nickel-based. While nickel batteries had to be charged for over 10 hours before first use (to prime the battery), lithium batteries only need to be charged until the mobile phone says the battery is full.
Don't let the battery run flat too often and apply regular charges
Another advantage of using a lithium battery is that it doesn't suffer from 'memory' -- a charging issue that affects nickel batteries. If nickel batteries are charged randomly and aren't fully discharged regularly, then large crystals will form on the internal cell plates, which causes it to underperform.
A lithium battery can be charged whenever it needs it. Full discharges are not needed -- indeed, they are inadvisable. Instead, let the battery run down to around a third of its capacity and charge it on a regular basis rather than running the battery completely flat.
If your phone is not displaying the correct amount of energy left in your battery, then let it run completely flat and this should solve the problem.
Keep the battery cool and don't leave it in the sun
Lithium batteries come with built-in circuitry that protects them from overheating, so leaving a phone to charge overnight or over a couple of days won't damage it. However, if a lithium battery is over-charged for too long (eg a week) it will start to heat up. Overheating is a lithium battery's main enemy and could result in permanent damage to the battery. The battery can also get overheated if your mobile phone is left in the sun or near a radiator, so avoid doing this as well.
After about two years buy a new battery for your mobile phone
A lithium battery only lasts for around 300 to 500 charges, so if you have been using the same battery for more than a couple of years you may need a new battery. Buying a spare battery is not advisable unless you use it on a regular basis because lithium batteries deteriorate even when they're not being used.
