I use lots oi different paints, Citadel, Army Painter, Vallejo... There are loads of them!
The one thing they all have in common is that they use a clear gloopy substance and mix it with the pigment to get you your nice painty goodness in a pot. ALL will split and separate out and need a mix before use.   
How do you get a good mix, especially with dropper bottles?

BALLS!
The image above is of some tiny glass balls called paint agitators; you get a bag a 100, 200, 50 - whatever you want from places like eBay for next to nothing.

But what do I need these for I hear you cry?
You are going to add 1-2 balls to your paint pots. They will agitate the paint mixing when you give it a shake (much like a can of spray paint) this will ensure the gloopy and the pigment mix really well together giving you a good solid colour and consistency every time!



These balls work really well in thick and thin paints. I always start with one ball in a pot. Then, if it needs it, jam a second one in. There is no real guide on the thinness/thickness of the paint to how many agitators to add. In my experience it is best to judge on how well it mixes.

Agitator balls come in metal and glass. The metal ones are heavier, and I've not used them, so can't comment on how good/bad they maybe. I use the glass balls, despite being made of glass, they have a good weight and give a really good solid mix to the paint. 

I find adding agitators to the Citadel Contrast range mixes them to a very good consistency, and if you use Army Painter, agitators are a must due to the way Army Painter makes their paints.