Green pepper has a more bitter taste than red or yellow pepper.
Are there tricks to remove or diminish this bitter taste? A special way of cooking, or adding a bit of sugar or something maybe?
Green peppers are green because they are unripe. Unripe fruits and vegetables are naturally more bitter and less sweet than ripe ones. By far the easiest/laziest path is to just use a red pepper. It's essentially the same food, just ripe.
Sugar won't really do anything to the bitterness other than mask it. Salt will. Khymos has written that in parts of Asia, unripe fruit is commonly eaten with salt, salty spices, or soy sauce. These are all excellent options for peppers, as they reduce the bitterness and enhance the taste.
Finally, cook at low heat if possible. Pan-fried peppers tend to be a fair shake bitterer than slow-grilled or oven-roasted peppers.
(P.S. Removing the skin obviously works as well - I personally prefer them unskinned, but if you want to skin them, have a look at this Chow video on it - it's simple to do, you just char the outside and shake it in a plastic bag.)
Also, the interior white part and skin (not the big obvious part, but the sort of interior skin of it) are rather bitter also. Remove it be sliding your knife carefully and parallel to your cutting board slowly and shallowly.
Much of the bitterness comes from the skin. The skin can be removed with a peeler, or by roasting the pepper until the skin starts to release.
Well, the obvious answer is to add a little sugar, though this probably won't make much difference to the actual pepper itself. If you don't want a bitter tasting pepper, why don't you just use a red or yellow one in the first place?
I cook the pepper using olive oil and don't get that bitter taste. It also could be the combination of seasonings that I use with the olive oil. I have found that Mrs. Dash Tomato/basil/Garlic seasoning does wonders with the green bell pepper.
Cook the green peppers in water and add a some apple cider vinegar or lemon juice. Cook for about 5 minutes and then take them out of the water and stuff them as you usually would. I remove the insides and white skin on the inside of the peppers before boiling.