Stellaris

- Keep an eye on your neighbors. If one of them has an aggressive personality like "Fanatic Purifiers" you need to ensure your military strength is about equal to theirs in the diplomacy screen.

- Vassalization is a really good way to boost your military power. Vassals create fleets that will follow your largest stack in wartime. At the least, they'll act as cannon fodder.

- The AI is kinda bad at managing planets right now, for your sectors and for other empires. When you use sectors, you might want to build planets up before turning them over.

- On sectors, there's not much benefit to having multiple. Put as many as you can into one mega-sector and give them a 5 star Governor. (Sector govs don't earn XP)

- Torpedoes seem to be pretty good.

- 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. The early game you'll probably be feeling more of a mineral squeeze but later on, you'll need energy more, probably, so when you feel like you always have more minerals than you know what to do with, it could be good to replace mining buildings with power plants.

- Spaceports and spaceport upgrades boost naval capacity so you should consider building them even in sectors and upgrading them if you need more naval capacity.

- It can very hard with no espionage mechanics to know how to counter enemy fleets so having a much larger fleet that is more general is the safest bet.

- Upgrading large fleets takes much less time if you split them up and send them to separate spaceports to be upgraded.

- Spaceport buildings with ship bonuses only affect ships built there so it can be a good idea to have all the +speed and +evade buildings on the same spaceport that also has corvette assembly yards so you can build tons of speedy dodgy corvettes quickly. Having them spread across multiple spaceports can dilute their effects and make it harder to keep track of which spaceports are good for what.

- When colonizing, consider what systems you'll get in your influence more so than the planet itself. For instance, a nine space planet that'll get you control of some nice systems with strategic resources or whatever is better than a twenty space planet that won't get you anything new. Frontier outposts can be used to grab areas you'll want to build up later but you can only support so many of those at once. Once space fills up, you'll be able to colonize and develop the other worlds in your space, but establishing a larger zone will be good when all the space has been taken. Then you fill out your space.

- On that note, keep an eye on other species that you end up with control over and figure out who is good for what. A very strong species can be good for building armies, ones with different preferences are obviously great for filling out your space. There came a phase of my game where I could make a tally of all the ocean worlds, arid worlds, etc in my space and I would go to one world of ocean people that I had captured, make six colony ships, make a thing of synths so that I could build ships of synths to send to tomb worlds, etc. This brought me to a much higher power level and naval capacity without having to fight wars. Diversity helps. If you don't have other species, then genetic modification can help you get the right preferences.

- If you purge an entire planet for whatever reason, leave one pop unpurged, resettle a pop of whoever you want on that planet, then purge the last native pop. Empty planets are just that, empty, and I had to build a new colony ship and actually lost a gaia world to an AI because they colonized the system before I showed up. Alternatively, Frontier outposts can be used to maintain control but plan for it.

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

- Techs get much slower if you have a lot of pops so try and keep up with building and upgrading science labs.