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There are several ways to dry & pin alcohol-stored material. Freeze-drying and vacuum-freezing gives almost perfect results, but it is expensive and time-consuming.

As a quick&dirty alternative I have used a acetone bath. Specimens are removed from alcohol, pinned while wet, and then immersed in acetone overnight. The time isn't critical, a few hours is enough for small species and times up to several days cause little apparent harm. Later the flies are removed from the acetone. Ay this time the wings of many smaller flies will fold; resting the wings against some thick paper and carefully moving it will usually open the wings. The paper should not be very absorbent, otherwise it will drain all acetone away before you have time to sort out the wings.

There also downsides: acetore-processed specimen are often slightly paler than natural and very brittle. I do not know how well they keep with time; the oldest syrphids processed this way are about 5 years old and they show now long-term harm.

A few hours in acetore will also help preserving colours of many semi-transparent yellow diptera like Lauxaniidae, Drosophilidae, Syprhidae. I do this regularly with nearly all yellow species if I have the time. An acetone bath will totally stop the darkening from from many syrphid suffer during drying especially the handsome Chrysotoxum are prone to turn almost black :(

If you are interested in more details, I could prepare a article on this site with photos of the equipment I use for this purpose.

EDIT. Oh yes, you need to use fresh acetone. As soon as it turns slightly yellowish from grease, fine hairs will start to stick together when dry and you'll get ugly specimens.