Analysis (Note: If you understand the bullet items above, this
section can be skipped.)
The middle game creates possibilities. Through drawing and trade you will be exposed to many cards. The trick is, any really good cards you play will immediately become a target. So, you want to hold some of your best cards until the last turn so that they are only a target once.
Your first goal should be to collect cards that allow to move rapidly, and in unexpected ways, during the last turn. It is common near the end that many players will be in a position to win. If you play a "normal" turn during the last turn you might discover a P2 or P3 planet, you might capture a P2 or P3 planet, and you might lose a planet as well. Your net gain: roughly P2 or P3.
Now, let's say you have been stockpiling cards. You have saved a really good planet until the end, so you discover Eden (P4), you have also saved a Fast Scout Ship card so you also "discover" another P3 planet (or better), you play 3 Civilization cards that boost your Population Potential, you take a planet and you lose a comparable planet. Your net gain: roughly P10, a huge difference. If you had a Temporal Rift card you could discover or capture even more.
It is important that the example above should happen during the last turn. If you have guessed wrong and the game is not over, you will lose many of those wonderful cards the next turn when the entire board gangs up on you.
Staying out of the lead is important. Often, players have many seemingly equal choices of who to attack. When in doubt, they will usually attack the leader, so it might as well not be you. If you are in the lead, be quiet about it.
If everything has gone as hoped, you will be in position to win during the last turn. At this point the remaining players fall into two categories: players that can win, and players that can't. Although that sounds obvious, it is important to realize that the trailing players may decide which player ends up winning.
So, you want to use the middle turns to invest in some friends. Help people that you think can, and will, help you later. Conversely, don't pick on the same player all game because they may be looking for revenge on the final turn.