I use those 32 fluid ounce (almost 1 liter) containers sold in the US for fruit fly culturing. I take an empty one and put some vitamin or whatever powder in the bottom, then I take a fruit fly culture, tilt over about 120 degrees and open the lid a little while over the mouth of the empty container, then I tap the culture vigorously to get flies to fall into the empty container with the powder. As I do this, occasionally I tap the "empty" to stop the flies crawling out. Once I have enough flies I reseal the culture while it's still over the "empty" and then I put the culture back on the shelf. I then swirl the flies in the "empty" with the powder until they get a nice dusting, then tape the "empty" a little to get flies into each terrarium. I rarely lose a fly during this whole process. It does take a bit of practice if you've never done it before.

One tip is that flies move faster the warmer it is, so it may be easier for you if you cool down the culture a few degrees before getting the flies out.