Sometimes you can squeeze a bit more life out of your almost dead battery. There is no real way of making them good as new but sometimes you can get just a bit more life from them when you need them.
1. When your battery gets to very low voltage, a lot of smart chargers won't know what to do. Sometimes they can't recognize there is even a battery attached. If you take an old school manual battery charger and give it a good blast, it can break the resistance and allow the battery to take a charge again. Think floor charger set on high.
2. You can also bang the battery on cement. As crude as it sounds, sometimes it will knock some crud off the plates allowing a bit more usable surface area on the plates to charge.
3. If you only have a smart charger, but you do have a known good battery, you can wire the bad battery in parallel to it and the charger will recognize the good battery and charge it. Then the charge will flow through the jumper cables that are wired in parallel (red to red and black to black) and essentially trick the charger to charge the bad battery that is wired after the good battery.
As always batteries can be dangerous. Take the lids off if they are not the maintenance free kind. Make sure there is plenty of ventilation. Make sure you wear safety glasses and do not let the battery charger stay on high for too long.
These tips are meant to try to get a bit more from your dead battery when you have to, they may not work, but at this point, what do you have to lose?