Another old, well-known exploit not listed here: L&O players who go on conquest sprees have a major advantage when they redesignate every single captured planet as a sector capital right away. These will immediately start producing imperial guards, discouraging them from going into civil war and making them harder to recapture.
If every captured planet is a sector capital, losing one to enemy reconquest won't result in other planets defecting as long as the captured planets are far enough away from the player's original planets. This element of the strategy works under any doctrine.
There's an efficiency hit associated with this strategy since the player has to redesignate the planets again once the war is over, but it's not as bad as the losses that would result from having the planets go into civil war.