Heat equal parts water and sugar in a saucepan. Add a lot of extract, and let reduce until syrupy -- but not too syrupy, because then it doesn't mix as well. Pour into bottles and let cool.
Mix syrup with cold fizzy water for Italian soda, or cold fizzy water and milk for French soda.
We liked all of the syrups both plane and mixed. Some of our most winningest combinations follow, but everything was pretty much delicious with everything else:
fizzy + milk + mint
fizzy + milk + almond
fizzy + milk + orange + anise
fizzy + raspberry
fizzy + raspberry + lemon
fizzy + mint + lemon
These were really excellent. You should probably make them.