#3 Defend with one ship, wherever possible
As long as the tech difference is not large, defenders fire first. This means that you get a guaranteed 2:1 or 3:1 or whatever loss rate against an attacking enemy. This is particularly nasty at high Weapons levels, where you can get a 25:1 or more loss rate. If you’re on the retreat, a series of one ship defensive stands can really whittle that pursuing enemy down.
As a corrolary, you need to constantly be on the look out to get the best loss rate possible. Because of the way the combat model works, you’ll never be able to eliminate an enemy fleet at little or no cost to yourself, given roughly equal tech levels. Likewise, having a huge size advantage over the enemy doesn’t reduce your loses in the slightest.
So let’s say you’re facing a cocky attacking fleet with 100 ships that’s coming down a line of your stars and you only have 50 ships to defend against it. You both have Weapons Tech 10. If you make a bold stand at the first star, you lose all your ships and the attacker loses 55 and gobbles up the rest.
However, let’s say you leave one ship at the threatened star and drop back: the attacker now loses eleven ships to your one and comes out of the battle with 89 to your 49. At the next star, you do the same. He now has 78 and you have 48. Better still, you’re probably falling back on your supply lines while he’s outracing his. Sooner or later, you’ll be able to take him out.
You can thus trade stars for ships very effectively and should never make a “last stand” defense unless there’s a pressing reason to do so (i.e., you need to attrit the attacker as much as possible so he’ll be vulnerable to an immediate counterattack).