Trickle charging is supplying a battery with just a small charging current that can be continued indefinitely without overcharging it (I do not believe that NiMH should be charged this way - in my experience it shortens their life. The older NiCd could handle it and could be left on trickle charge indefinitely and people assume that NiMH is just the same. However, everyone seems to like to trickle charge NiMH so maybe I'm wrong.)
There are some circuits down this page for charging a 12V NiMH pack:
Charging NiMH Cells
But...
Be aware that NiMH cells are charged at a roughly constant current, that means the the voltage provided by the charger will be more than the battery being charged, in order to push the current into the battery.
If you intend to charge the battery with the load attached, make sure that:
(1) the load can take the few extra volts without being damaged and
(2) that the charging circuit can supply enough current for both charging the battery, and powering the load, unless it is voltage regulated to just the right voltage to trickle charge the cells (rather than using constant current) and can provide enough current to power the load as well as charge the battery.
Alternatively, if you will disconnect the load while charging, then things are much more simple. You can charge the battery with the circuits shown on the website I linked at a normal charge rate. I think that will make your battery live longer than constant trickle charging.