Photographers get through AA batteries like they are pennies in an arcade. Unfortunately they are not pennies they are pounds so we get the re-chargeable ones and save a bit of money by using them more than once. That is the theory anyhow. In practice what I see is photographers with a bag of batteries with no idea of the charge state, age, condition etc. of any of the batteries. Hmmm, so much for saving money.
So, here is how I treat my batteries. I buy them in packs of 4 and usually with an included 4 pack holder. I buy good quality batteries (Sanyo Eneloop are a great example of a good high quality rechargeable battery) and I keep them in their original sets of four.
7DayShop (www.7dayshop.com) is a good source for the batteries and the cases. They regularly sell packs of four batteries with a case included.
The 7DayShop own brand batteries are pretty good too. They have one minor niggle which is not a show stopper but does make them unsuitable for some devices. They are slightly chubbier than a standard AA. It is probably a fraction of a millimetre but can be just enough to make them a tight fit in some devices.
I use the 2500mAh ones and have found them to be very good. Charge them as soon as you get them.
So, you have some good batteries and you pop out and use them in your flash and then what? First thing is to keep them in their original sets of 4. Four batteries go in one case and get used and go back in a case as a batch and get charged as a batch. This ensures that they age together and you do not mix up capacities when fitting to your device. As soon as you get back in start charging the used ones. The sooner you charge a discharged battery the longer it will last. Putting off recharging the batteries until you next need them shortens their life. When not in use they will slowly lose their charge. If you don't use them for a while (4 to 6 weeks) pop them in the charger to keep them topped up.
Now the biggy. Spend some proper money on a charger. The cheap chargers and a lot of fast chargers work by shoving a current into the battery for a fixed period of time. This means that some batteries (higher capacity ones) can be undercharged and lower capacity ones can be overcharged. Overcharging is bad for batteries just like it is bad for you if you do it to clients. They don't come back, they are no longer a client. Overcharge your battery and it probably will never be the same again. You will considerably shorten the life of the battery and this is when you hear people saying 'Those batteries are rubbish, only lasted 3 charges and then let me down'. No, the batteries were good, the charger was rubbish.
I am using Eneloops I bought years ago and they are like the day I bought them. I have 24 of them (6 sets of 4) and have never had one fail or not hold its charge. I also have 40 (10 sets of 4) of the 7dayshop batteries and have only been using these for around 4 months but so far not had a single failure.
The charger I use is a Technoline charger that I bought about 8 years ago. The model I bought is no longer available but there is a newer equivalent which looks and works pretty much identically. The Technoline BL-700.
Yes, it is a fair bit of money for a battery charger but mine has paid for itself many times over by not killing batteries when charging them. If I am in any doubt about a battery set I run the refresh program which invariably brings the pack back like new. Rarely had to use it and cannot remember the last time I did (and never had to run it on the Enelopoops) but every time I have it has been successful and brought the dead batteries back to life (which is probably why I am writing this at Easter).
Decent batteries need a decent charger or you might as well buy cheap batteries and throw them away once you have used them a couple of times.