I ALWAYS made a pitcher full of formula, and then filled 8 or so bottles at a time. The formula lasts pre-made at 2-3 days, so based upon how much lo was eating at the time, I'd make a pitcher of 2 days worth.
The best thing I found was to make sure my water was either room temperature, or boiled and then cooled (but still a bit warm). The WORST to try to mix formula with is cold water!!!
I'd put the number of scoops in a pitcher I needed, then the water, and then mix with a whisk. It seemed to make it MUCH easier to get rid of those lumps!!! Also, even though the whisk puts air in it, since the formula is in the pitcher, it has time to settle. I always let it settle an hour or so in the fridge, then used a regular spoon or even the whisk again to restir, then pour 8-10 bottles. It made for MUCH less dishwashing at my house!!!!! I wouldnt have it any other way!!!