Ideally you would use very fresh milk where the cream has not separated yet. I have absolutely no chance of that, so I have tried lots and lots of different things. By far the technique that has worked the best for me is to take the milk out of the fridge and shake it every 10 minutes or so until the milk gets to room temp (about 20 C). You don't want to shake it past about 22 C because it does seem to damage the fat globules (I get more fat coming out in the whey). At around room temp, though, it seems that shaking works incredibly well. I've also tried things like blending and whisking. These produced very poorly performing milks. For whatever reason, the fat just doesn't disperse properly and I end up with very small chunks of fat. With shaking I can literally get a 2 inch plug of cream reincorporated in the milk such that I can't tell the difference between that and fresher milk.