Three most important things to ensure your batteries go the distance:
1) Sizing. Your battery bank must be correctly sized for your system and your system for the battery. If you demand too much from them or charge them too little they will overheat, discharge too far or sulphate.
2) DOD. Depth of Discharge. Manufacturers will tell you that your batteries will last for 'X' cycles at 'Y'% DOD.If you discharge them too much ('y') before recharging then your 'x' reduces which reduces their lifespan. Which really leads into...
3) Charging. A normal $100 charger from repco is great for your car battery if you are going to just slap it on and ram as much energy into the battery as possible. Hey, if you forget it and damage the battery then you just go buy another one right? Solar batteries require a very specific charging regime. There are many different important stages in the charging cycle that ensure your bank of batteries stay in tip top condition.