I have fallen in love with raw milk. It tastes so much better than processed milk. Raw milk is not pasteurized or homogenized. Because the milk is not homogenized, the cream will separate from the milk.  Also, pasteurization or homogenization changes the flavor of the milk so the milk does not taste as good.  Homogenization breaks up the fat cells.  I buy three quarts of milk at a time. As I use the first quart, the cream is well mixed in the milk. By the time I open the second and third quart, the cream has separated from the milk and settled at the top of the bottle. How do I keep the cream from separating? I suppose I could shake each quart each day, but that is more trouble than it is worth.  Once the cream has separated, it requires a lot more effort than shaking to get the cream to mix back in.  I have to stick a knife through the opening at the top and break up the cream.  Then, I have to shake vigorously. 
A very quick option is to buy less milk at a time - unless there's a very specific reason to by in bulk, picking up a quart only when you're ready to use it may solve the problem, since it seems mixed when you buy it.  Additionally, raw milk has a shorter shelf life than pasteurized, so buying fresh may be better anyway.  If you did have a reason to buy in bulk, you might try mixing the whole (separated) quart when you first open it, and perhaps every few days when using.  Shaking may take more effort, but pouring into a container and using, say, an immersion blender may very quickly mix the milk well enough to keep for a few days while using.  If you get hold of a little milk frother (works kinda like a very very tiny immersion blender) it may be useful to mix the milk still in its container, at least as long as the liquid is high enough to be reached.  Another possibility, one I've no idea if it will work or not, is that depending on the shape of the milk containers, you might be able to stand them on their heads, say, every other day.  Cream rises upwards, having that upwards change direction every so often might keep the cream in suspension longer without requiring a lot of physical effort.  I've heard it works to keep peanut butter from separating (even on longer timescales, flipping once per week or month), but then peanut butter is so thick and the timescale it takes to separate so much longer I'm not sure if milk will work the same way.  I don't recall how long this kind of mixing will stay un-separated - I tended to shake just before use and that was effective enough for me - but it may help, even if you have to do it periodically. 