The game is all about ship production. Try to get as many planets as possible as fast as you can in order to build more ships than any of your opponents.
Do not attack a planet with less than 10 ships, unless you can conquer it.
Attack neutral planets before other players' planets.
Split your ship storages across several planets.
Plan your attacks in advance.
Use the ruler to find out the distance between planets.
Use the information window to get information on planets before an attack.
If this information is available to you, it is preferable to attack a planet with a low kill ratio from a planet with a high one.