Instead of using the tip of a knife, use the heel to wedge into a corner of two slices that are frozen together. Give it a little jiggle to get the knife's heel deeper in between the slices, and they should pop apart, ready for thawing or toasting.
The easiest way to get single slices is to freeze them individually. Spread the slices out on a baking sheet, put it in the freezer for about an hour, then package them in individual airlock plastic bags.
The concertina method involves cutting a sheet of greaseproof parchment paper into strips. Place a strip between each slice of bread before freezing to keep them separated.
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