You could use a soapy cloth to wipe the leaves with, then spray it occasionally with a mist of one part alcohol to three parts water.

Babywipes work well. They were recommended to me by a specialist citrus grower and I use them on my potted lemon.

Spray them off with jets of water?

And, don't forget to follow up with the insecticide treatment, as scale has several life stages. I like horticultural oil or neem.

GB

I don't think scale would even budge from a blast of water. They must have some kind of glue in them. I think these were dead scale because I just used a moist cloth and they came off very easily except some clear ones. I needed to get it done so I could get it into the greenhouse. No room inside for it out of Zackey's (the cat)reach.

This worked for an Olive and two Citrus trees.

1 capful of Fish Emulsion in a 16-20oz bottle mister filled w/water. Shake thoroughly. Spray leaf tops and undersides, branches, and trunks.
You might need to apply twice depending on amount of scale.
Discard left-overs. Toni