How can I make my tea stronger?

I know it sounds like an obvious question.

At work, my colleague has a drip-filter machine he uses to brew his tea. He puts about two table spoons for an eight cup pot.

This makes nice strong tea.

I use a glass infuser teapot, which I put two teaspoons in for about a mug of tea.

This has the advantage of being easy to clean up, and convienient. However, it's not nearly as strong as the drip tea.

How can I make my tea stronger? Do I simply need to put more tea in?

Nb. I'm talking about for black tea.

For green tea, I'm putting in 1.5 teaspoons, and adding a little cold water first, and that comes out fine.