[dogscoff]

1- select a planet deep in your terrirory that you don't need very much.
2- Trade the planet to an enemy AI with more favourable reproduction rates than yours.
3- Wait a while
4- reconquer planet, reclaim population.
5- Repeat as necessary.

You can also pick up some free facilities and units this way, as well as whatever you traded the planet for, but be careful not to let the enemy build a spaceyard on it and launch ships in your homesystems. (either pre-fill the facility slots, or re-conquer before he gets has time to build & use a spaceyard).

Bit of an exploit but hey- that's what the AI is for!