I make elderflower cordial; it's wonderfully refreshing in the summer.
As Julia says, there are lots of variations on the recipe. I boil the water with the sugar, then add the flower heads to the water once it's cooled down. I also add a load of slices of orange and lemon. I let that soak for 48 hours, stirring occasionally. To preserve, I pour white wine vinegar in; you can't taste it.
You could not preserve it at all and it will keep in the fridge for a week or so.
Incidentally, I wouldn't use any flowers that are by a road - they absorb fumes. I get mine from along the Isar.
Edit: Oh yeah, I'd forgotten that the Germans like to deep fry them. It ain't so healthy but tastes delicious.