Stellaris

Note: The following advice is accurate as of 2.0 and assumes the presence of all major DLC.

• Even basic Outposts can have some Defense Platforms built around them, handy if some small-scale nuisances like pirates keep up gunning for the same system.

• Don't forget to make claims on enemy systems before declaring war on them. Claim costs increase the further the claims are from you.

• Pirates pop up in unclaimed systems adjacent to your claimed systems, an incentive to eventually fill in any unclaimed bubbles left over in your territory.

• Use Starbases to block strategic chokepoints and to guard your borders. Border Starbases should always be fitted with a Listening Post which allows you to track the other nation's nearby fleet movements - once your borders expand, you can replace it with something else.

• Grabbing more territory will increase the costs technologies and traditions, with colonized planets increasing the costs a lot more than a system. Small and poor planets may not be worth colonizing if their output isn't worth it.

• Prioritize grabbing systems with large colonizable planets, systems with high resource quantities, and systems that make good hyperlane chokepoints. Even if an otherwise good planet is ill-fitting for your species, it can later be terraformed to make it more habitable.

• The general agenda of other empires can be deduced by their trait description. One who's a "Spiritualist Seekers" will like other spiritual empires, and a "Fanatical Purifier" is probably going to try and kill you at some point. Gifts and deliberately favorable trade deals are usually the only way at first to improve relations, until you can get a treaty which starts building trust.

• When you start making Sectors, look at their planets and see if they lend themselves to a specialization rather than just generic resources. They may have really good energy planets for instance.

• Lasers beat armor, kinetic weapons beat shields, missiles beat a little of both and are more accurate, but can be shot down by point defense. Normal guns and lasers are not point defense.

• Energy production is important, even if you have a good surplus throughout the game, a sizable late game fleet can put you in a pretty big deficit if you don't have a large production. Keep your fleets docked in a Starbase with Crew Quarters built when they're not in use.

• If you do find yourself with an overabundance of Energy Credits, they can be used to terraform, or to purchase the services of the various Enclaves (Curators, Artists, etc.).

• Research is stored when your scientists are busy with special projects so don't be concerned about falling behind on tech due to projects.

• Look through the Ascension Perks tied to your traditions at the start of the game, and try to pick a few to go for. Some are locked behind tech, so it's helpful to decide which type of endgame you want to pursue so that you can pick the techs as they come up.