A good technique to make Iced Tea

Have you tried using one green tea bag and one bag of herbal tea? There are many herbal tea sampler packs that you could get to try several flavors of tea. Since you are making iced tea, I would also recommend adding chopped fruit once the tea has chilled - I would imagine some crisp apples or even a fresh melon would add a really refreshing profile. You can even let the fruits steep in the chilled liquid, like a tea-sangria.

Another option is to create a flavored simple syrup in order to sweeten your iced tea. It's always better to use a simple syrup in a cold liquid than any kind of 'dry' sugar. Simple syrup is easy to make on the stove or in the microwave, and just add some flavors you might like while heating it. I wouldn't recommend vanilla for green tea (gut reaction), but maybe lavender, orange or cardamom?

The amount of tea you use won't affect the flavor of tea adversely; the way you brew it will. This also depends on what kind of tea you're making (black, oolong, green, white, herbal). Greater quantities of tannins are released in higher-temperature water, which makes the tea more bitter.

TL;DR: If you follow the instructions for brewing temperature and time, you should be able to more or less add as much tea as you want.

Black teas are brewed with boiling water, typically for 2-3 minutes. Green, white and oolong teas are usually brewed at lower temp (180 F) for 3-4 minutes, and herbal teas at just below boiling for anywhere from 4-8 minutes.

I've found that I can easily more than double the amount of tea leaves I use, brew it at the recommended temperature for the recommended time (most tea packages should have instructions), and store the 2x concentrated tea in my refrigerator. Upton Tea has a great guide to tea for water temps and amounts. They also have instructions for hot and cold brewing iced tea - they recommend double or tripling the leaf quantity when hot brewing, then diluting to taste. (I buy a lot of my teas from here, and their instructions have never steered me wrong.)

If you're looking for even more information about preparation of and descriptions / histories of different types of tea, I recommend the Harney & Sons Guide to Tea.