A fascinating read on making simple syrup that is shelf-stable, and some surprisingly fascinating food safety lessons: http://cocktailjen.blogspot.com/2007/04/not-so-simple-syrup.html

In short:

Stir sugar into water in a 1:1 ratio
bring to boil
Add a dash of cream of tartar
simmer for 15 minutes
Let cool. If it hardens, add more water, repeat.

You now have shelf-stable simple syrup.

Add a shot of neutral spirit (vodka, etc) at the end, after removing from heat, to help preserve these syrups.

Add rosemary: 3-4 sprigs fresh rosemary during simmering

Add cranberries: 1/2 bag (1.5 cups) during the simmering process, let drain out.

Add orange peel!

Add mint: 15 cups packed fresh mint, drain.

Black Pepper: toast 1/4 cup whole black peppercorns, 1/8 teaspoon red pepper flakes, then add the syrup ingredients. Drain when finished.

Lemongrass Syrup

This syrup can be used to make soda or as its own for cocktails.

1/4 cup lemon zest (from 6 lemons)
1/4 cup lime zest (from 8 limes)
20 kaffir lime leaves
2 stalks lemongrass, roughly chopped
3/4 cup Simple Syrup (click for recipe)
6 cups soda water, divided

Combine citrus zest, lime leaves, lemongrass, and 2 cups water in a small saucepan. Bring to a boil, reduce heat, and simmer for 5 minutes. Strain lemongrass mixture into a jar and chill. Fill a 12-oz. glass with ice cubes. Add 2 Tbsp. lemongrass mixture and 1 Tbsp. simple syrup. Top with soda water (about 1/2 cup) and stir to combine. Repeat to make 11 more sodas.

Via Bon Appetit, http://www.bonappetit.com/recipes/2011/05/lemongrass-lime-leaf-soda :

"Chicago chef Graham Elliot uses aromatic kaffir lime leaves to make this sparkler from his sandwich shop, Grahamwich. They're sold frozen, and sometimes fresh, at Asian markets. If unavailable, substitute an additional ½ tsp. lime zest for each lime leaf. Lemongrass can be found in the produce section of better supermarkets and at Asian markets."