Since I have been into hobby-class R/C for a little while I've learned that peak-detection chargers are by far the best. These types of chargers charge at a constant current (voltage will vary) and takes advantage of the fact that the battery charging voltage will drop a little at the end of a full charge cycle. They see this drop (usually .2v or less) and trigger the end of charging. The MRC Superbrain 959 (**broken link removed**) is an excellent, relatively inexpensive charger that can charge from 1 to 8 cells of NiMH or NiCD batteries, with an adjustable current, peak detection threshold, and/or time limit - or use the "Set and forget it" option and it will pick the best settings automatically. It can be plugged in to house 120v AC or use a 12v car battery if desired. I got mine specifically for R/C charging needs, but I've since made some adaptors so I can charge all the batteries in the house and have thrown away all those crappy wall-wart chargers. Incidentally, I've taken apart a couple of those cheaper chargers and all they had for "circuitry" was a small AC transformer with two diodes for full-wave rectification directly hooked to the battery terminals. Ugg.
Generally, a higher charge rate of less time will give the battery a higher voltage peak for more "punch", while a long, low current charge will provide a longer run-time, but with a slightly less battery voltage. For example, when I charge my GP3300 sub-C cells (3300mAh), I charge them at 4.5 amps for as long as it takes (about 20-25 minutes). This will give my 7.2v (6 cell) packs more punch for electric motors. But, when I charge my receiver pack (6v 1100mAh), I want a longer runtime so I charge those at .5A or 1A. No matter what, a peak charger will charge them only as much as they need without overcharging or undercharging. When charging, the battery should be a little warm at the end of the charge. NiMH cells tend to be more sensitive to high temperatures, so it's best to keep them from getting too hot. NiCDs are more forgiving, but they generally don't have the high capacities found in NiMH cells.
Hope this helps...
[Edit]: I almost forgot: It's not good to run NiMH cells down until they are totally dead. I've read this causes "cell-reversal" and may render the cell unusable. For best results, NiMH cells should be discharged to no lower than .9v per cell.