Is there a way to extend NiMH units battery life?

I have a dozen NiMH 3.6v (3x AAA) 800mAh battery packs
that don't seem to charging well just wondering if
I can do anything to kick a little more life into them.

I have a varible 24V DC power supply but dozen't have a varible
current control is there a way of charging them with this?

What voltage should I charge them at?