The care and feeding of acrylic paint (regardless of brand):
Stirred, not shaken. Hobby paint bottles don't have sufficient volume for shaking to be effective. There are exceptions, such as Hawkeye's Talon metallic acrylics, which should be shaken. Shaking also entrains air into paint, and air is not good for liquid paint.
Whenever you are done with a bottle of paint, thoroughly clean the rim, inside of the neck (throat) of the bottle, and the sealing material inside the lid. That mean no paint anywhere on the sealing surfaces. Cleaning the throat keeps dried and partially dried paint from falling into the liquid, where it will promote polymerization.
Never thin the paint in the bottle. Decant some and thin that for airbrushing. Never paint directly from the bottle, either, especially with Tamiya or Gunze.
When brush painting Tamiya, dip the tip of the brush in 90% isopropyl or Tamiya acrylic thinner before loading it with paint. Keep the brush well loaded with paint, paint with a wet edge—you flow the paint on, more than spread it. Don't worry, it will snuggle down to the surface as it cures. Never go over a painted area until the paint has fully cured (overnight as an absolute minimum.)
Finally, what you use to stir the paint is important. A toothpick won't do the job. You need something paddle shaped. I flattened the end of a steel wire, filed and sanded it smooth. This spatula has served for years, is easy to clean, and disperses the paint quite well.