I picked up a used paint shaker. It was used by a 'nail tech' at a beauty salon to mix fingernail paints and varnishes. You can also look for similar shakers marketed towards the tattoo artists for mixing their inks. They tend to be somewhat cheaper than the full on lab shakers.

I have found though that even Vallejo and similar eye dropper paints may still require stirring. To that end I took an old battery operated vibrating toothbrush, removed the brush head and glued in a length of metal rod that fits through the opening in the eye dropper end. It does a pretty credible job of stirring up the paint settled at the bottom. I then use the shaker to give it a good mixing and let it sit for a few minutes to 'settle' the paint before using.

I use a Badger paint mixer for all of my tins of Humbrol and glass jars of Testors, Tamiya and Gunze paints. I take a section of a paper towel tube to put around the bottle to cut down on paint spray while stirring.

These are the methods that work for me. You can find a ton of modelling hacks over on Youtube. You'll just have to find what works for you.