Build habitat structures whenever they're available. These raise a planet's max population, which means more labor. Don't build them if you are already having trouble supplying a planet with resources; they are also not strictly necessary on foundations and trade hubs. Habitats take a while to build. The largest hab structures for hazardous worlds are arcologies, available at TL 7. For habitable worlds, the largest are planetary arcologies at TL 8. Planetary arcologies takes 4 days to build! At 100% efficiency, a TL 8 habitable world with a planetary arcology will produce about twice as much labor as a TL7 hazardous world with a regular arcology.
Keep an eye on actual unit production through the “production” tab. The “structures” tab only reports how many units a world is trying to produce. If your planet is trying to import more resources or ship components than it is getting (indicated by a number in brackets on a given resource of component in the production tab), your world is wasting labor. Either switch to cheaper unit types, move labor into defenses on the unit-producing world (defenses are cheaper than units), or add more resource/autofac worlds.
You will need a lot of chronimium once you start building ships that are TL 8 and higher. The TL7 Victory-class starcruiser and armored constellation defense structure also require chronimium. Securing abundant chronimium planets is really important if there are any nearby. Chronimium uses a lot of trillum to produce, so keep your chronimium processor planets supplied by your trillum extractors.
Pay attention to what kind of ships your neighbors have and build or station proper deterrents on your most vulnerable planets accordingly. Build citadels to deter starship attack and take out nearby jumpship yards to prevent them from attacking you with jumpships. If enemy jumpships are able to operate inside your borders, your empire will be very vulnerable.
If a planet imports a resource from two planets, it will default to importing 50% of its need from each. You can manually change the allocation by clicking on the trade route; if one planets is producing much more than another you aren't forced to draw equally from both. For example, if planet Trillum A produces twice as much as planet Trillum B and your capital imports from both, you can change the Trillum B route to only import 33% of the capital's need and the Trillum A route will change to 67% by default. Once you manually change a trade route it will stay at the assigned % forever, so if you want a trade route to start auto-balancing again you will need to cancel and recreate it.
Armored infantry are a lot tougher than standard infantry and you'll need fewer transports to move them around. Building fewer transports lets you build more attack ships. When your resource supplies seem stable, set up an infantry academy and an infantry autofac. One autofac can fully supply one academy that's building 100% armored infantry, provided that the worlds have the same labor output. You must manually order the academy to build 100% armored infantry, otherwise it will build 50% regular, 50% armored. If both your autofac and academy are TL7 or higher they will also try to build exotroops. Don't do this right now, exotroops are costly to build and require chronimium and >1 autofac per academy. However, they are verey effectie
Don't leave your infantry on a newly-conquered planet unless you are expecting another player to attack it. They won't help with the “industry demands more military defense” message (only stationing ships or building defenses stops this). Use infantry only to attack, to garrison vulnerable planets against other players' attacks, and to suppress civil wars. Transfer infantry down to your capital when you're AFK so that they don't get destroyed in their defenseless transports by another player.
Foundations can raise other planets' TL via a trade route, and their TL will rise to match the capital. Foundations tend to produce a lot of TLs so it's safe to allocate labor to defenses on them as long as you ensure that the foundation is still generating a surplus of 10-20 TLs
It's always a good idea to build fusion programs on TL 5 worlds if you can't supply them from a foundation. Going from TL 5 to TL 6 results in an immediate increase in labor production without much or any population penalty. It also makes more habitat structures available, as well as powerful hypersonic missile defenses.
Don't raise every planet to TL 7 or higher right away. TL 7 is when planets start demanding luxury goods. At TL 10 demand for luxuries is enormous. You'll need a lot of dedicated consumer autofacs to supply a high-tech empire and have it remain productive.
Regions of space where worlds are densely clustered are more valuable than regions where planets are spread out. When planning to start a new sector capital (which will project , pick a planet that is near a few dense clusters and good planets with abundant trillum. A sector capital will be able to grow a region into an industrial powerhouse for your empire if there are a couple of independent earthlike or ocean worlds to be CG autofacs, at least two planets with abundant trillum, and a couple planets with at least major chronimium within a 200 light year radius of the sector capital or a hub that is supplying it. Worlds with abundant chronimium are probably the most valuable planets in the game; snag these planets whenever and however you are able.
If you are using trade hubs, put your hubs within 50 LY of capitals to ensure that every planet within range of the hub is under the capital's influence. This is not an absolute rule and some clever geometries use alternate spacing of hubs and capitals.
Eventually, build starship yards close (<50LY) to your capitals, hubs and foundations so that you can quickly reinforce these planets with heavy starships.