Buy a charger that's specifically for NiMH batteries and not for NiCd as they have slightly different charging protocols.
One of the best would probably be the
BQ CC63 8-cell charger from Panasonic which operates from the main's and is specifically designed for NiMh batteries.
It charges each cell individually so will not overcharge a cell as can happen if they are charged in series.
It may be a little pricy, but should be worth it if you are charging a lot of cells frequently, and what to get maximum life from them.