Some people like to blend the lassi with water instead of milk and flavor it with nutmeg and a hint of cayenne pepper, but I prefer not to.
You can decorate the top of the glasses with crushed nuts (slivered almonds, pistachios or cashews), just as it's done for other Indian drinks, such as sharbat.
I prefer to use superfine sugar. It's a fine-grained sugar and it dissolves much more quickly than regular sugar. You can add up to ½ cup of sugar to plain lassi, but it's not meant to be too sweet.
I used Greek-style yogurt because it's firmer and more dense than regular yogurt. I made my own yogurt last summer. It's easier than you imagine; making homemade yogurt doesn't even require a yogurt maker. You can click on the link for the recipe.
You can find rose extract at Indian stores. It's sold in small 0.7-ounce bottles.