Get Nickel Metal Hydride (NiMH) rechargeables rather than Nickel Cadmium (NiCad) - more capacity and reduced memory effect. Also more ecologically friendy when disposing.

Use a charger with a "discharge before charge" function as this keeps any memory effects to a minimum.

Possible problem could be that these sorts of rechargeables are 1.2v per cell not 1.5 and some units objects to the lower voltage. However you can get rechargable "alkaline" cells which run at 1.5v but I've never used these so I don't know whether they last as long as NiMH cells.

Another possibility is to get an external battery holder that takes a suitable number of "D" cells and send that in via the external power socket (asuming it takes DC). Doesn't do a lot for portability, unfortunately!

Hope this helps!