I have to admit, I've only actually made this once before. And, if I'm being totally and completely honest, I think I like the store-bought versions just as well, if not better.
From 660 Curries
1 tsp. tamarind concentrate
1/4 c. chopped jaggery (or firmly packed brown sugar)
10 pitted dates, chopped
1. Dissolve tamarind into 1 1/2 c. water.
2. Add the jaggery and dates and bring to a boil over medium heat.
3. Boil, uncovered, stirring occasionally, until liquid has reduced by half.
4. Remove from heat and allow to cool for 5 minutes.
5. Transfer to a blender jar and puree until smooth.
6. Store in fridge or freezer until ready to use. (Will keep in fridge for 5-7 days.)