Here's an optional rule we use to avoid someone missing out on fun, and to cause players to restrain themselves from "going for the jugular" against someone. Note this is only useable w/my free setup variant, though I suppose you could adapt it to the regular rigid setup:
If a player is eliminated, he comes back as a rebel at the end of the next status phase. Choose a random non-home tile w/planets, and the system rebels. Randomly select a race w/the same # of planets in their home system, replace the tile w/the home system tile, give the player his starting forces (no tach or TG) and let him rejoin the game.
Also, if a player has less than 5 units on the board and a majority of players including himself vote to allow him to suicide, he may do so at the beginning of the action phase then return at the end of that phase as above.