Every other day we do the following:
I gather all the bottles and put them in the sink.
I take the tops of each bottle.
I take the nipples out of each top.
I put soapy water in the sink for them to soak in.
Richard (usually) washes the bottles.
Then he (or I) puts them on the racks to dry.
When they are dry...
I put all 24 of the bottles back in the sink.
I fill each bottle with either 4 or 6 oz of warm water.
I put one scoop of formula in each bottle.
I put another scoop of formula in each bottle.
I put a third scoop of formula in the 6oz bottles.
I put a nipple in each top.
I put a top on each bottle.
I shake each bottle, setting it on a towel after its shaken.
When all the shaking is done, I check each bottle for chunks.
I shake all chunky bottles again.
I take a second towel and dry each bottle.
I put all bottles in the fridge.
I do this every other day. Three or four times a week.
All of this to say: I would gladly pay for a paint shaker machine if it had a bottle sized adapter. Or even pay an energetic 5 year old. Too bad all my friends kids are 2 or under.