Middle Game
Attack Points:
Try to find a weak and strong link. As in chess, you want to control the middle of the board/universe. In SC, you want to control the link that leads to your world AND leads to another player's world. Sometimes these are called bottlenecks.
Diplomacy
Making friends in an alliance game is important. But too many alliances spoils the fun. You can't protect an ally if you are also aligned with his enemy.
Overbuilding:
Overbuilding is an important tool in the game. As in poker, you have to know when to use it, when not to use it.
The idea behind overbuilding is to create a large fleet that will be at full strength by the time it reaches its target. Sending out one ship at a time will build tech, but it can't stop a single, large fleet. I may overbuild at BR2, but only once. It's a quick transition in many games from BR2 to BR3, and you don't want to be caught short when your enemy reaches a higher BR level. Normally, the fun begins at BR3.
On tech advancements, I often will use terraformer at the start. If I can successfully get a pop trick, I will be ahead in economy and won't have to worry too much about having to build a minefield for protect. So my next tech use will be a minesweep for a possible quick, early nuke.
If I fall behind in economy, then I will keep in reserve the ability to build minefields and satelites. When i do build a minefield, I build a BR1. This saves tech and works just as well as a bigger minefield, which are the last ship to die in a battle according to the original builders of the game.
You will need a minesweep against all but the lamest players. Any player who has played a few games knows that an attack force without a minesweep can be taken out with a minefield. There's nothing sweeter than taking out a huge fleet with a BR1 minefield while your own fleet continues on its merry way, nuking as it goes.
Limit Overbuilds
I try to limit how many times I overbuild. It's nice to reach the next ship strength before a foe. Also, when that fleet attacks other ships, don't build more ships. Let the update get your ships back to full strength. Learn to estimate when your ships will go from weak to strong, so that you can move into a system as the ships reach their maximum force.
End Game
Minesweeps:
As I've already said, you will need a minesweep (or two or three minesweeps depending on how much economy everyone has) to take out a foe. Sometimes you can send in a bunch of attacks first, if there is no minefield present. An enemy then might feel, hey, I can just build a minefield. That's OK if you are sending in a minesweep onto the home world on the next move. A sweep doesn't have to be included in the main wave if your foe doesn't have a minefield in place. But if a minefield is present, then you must send in a fleet protected by a minesweep. The advantage of leaving a minesweep behind is that it can't be destroyed in the initial attack. I sometimes will advance a single ship to the system I want to nuke next. I can set the ship to nuke while moving in the rest of my fleet, which will protect that single ship.
Diary:
Keep a diary of players you've met to one side. You can look up players and see how you did against them the last time you met them. Some players play each game differently. Others play the exact same way, no matter what. Some players like to align with everyone, others like to play to the death, taking no allies. If you play a lot in games that allow for allies, this could be a valuable tool. It might even help in grudge games, with no alliances allowed, in that you will know how a certain player plays.