[spare batteries] My recommendation is always get genuine batteries. ie the battery is same brand as the tool.
[extending the life of the battery] Keeping a battery out of extreme heat is probably the best thing you can do to help reduce the rate in which the lifespan of the battery goes down by. I like to at the very least keep my batteries in my insulated garage. I don't have the luxury of a climate controlled garage which would be even better. You could also take batteries inside the house if you had to. If your batteries are alreadey warm I would hold off on charging the batteries until they have cooled off. If the battery is too warm the charger should know not to charge the battery, activate hot/cold mode, and turn on the fan if the charger is equipped with one.
[end of battery life warning signs] The biggest sign of a battery being end of life is reduced maximum runtime. Another sign is the charge display on the battery might throw an error code. If it starts throwing an error code, it is probably time to think about getting a replacement.