I would agree with all posts so far. But simple ways to get to more astringency & bitterness is brewing tech. Try boiling water, extended brewing times, pre-heated vessels & serving cups.
Next best way is to do the unconventional and make it like cheap tea bags where they only have the 'fines', that gets you maximum extraction of everything inside a tea leaf. Break up your leaves, grind them to a fine powder and you'll get, most likely a too bitter brew and coffee like caffeine rush
Last trick is to use a very high leaf to water ratio for a extra strong brew. I made the mistake of filling up 1/2 way a 2 cup ceramic tea pot with inexpensive Baozhong/pouchong @$21/lb...greener style oolong. Then proceeded to become distracted and left the tea to brew for several minutes. Most ungodly strong/bitter brew evah! Had to dilute it with water in 1/2 and it was still the strongest tea I've ever had.
Strangely enough, I got the same style pouchong from Valley Coffee & Tea, spring harvest 2011 that costs $82/lb, so mid-grade, and that one is *much* less bitter, even when over-steeping. I know the owner of the store, since she drinks coffee, likes to take the most broken pieces of leaf from the bottom of the $21 bin, and brew up a stronger cup from that.
Both of those teas, even when over-steeped with a larger quantity of tea leaves to water, still come out only as a medium yellow/amber color, and always paler yellow when brewed with less tea leaf, thought they do seem to have a good amount of slight astringency from the seemingly higher caffeine content?
While I could not say from experience, it would seem neither of the above teas are heavily roasted, more likely only lightly roasted. That kind of greener style oolong though, if I drink more than 3-4 cups, tends to give me a little upset stomach that other oolongs do not.
You can go for the heavily roasted DC or Wuyi rock teas, but that flavor is stronger from the roasting, not necessarily from the tea, imho. A different kind of stronger flavor---the color of the tea is always medium-dark reddish
amber. I think I like both styles, many styles