seems like a pretty old post and you must have found your answer by now.But heres how I take care of it:
1)If you are buying an Indian Variety buy the smallest ones available. "Karela" goes bitter as it ages ( and grows in size).
2) For preparation I have two methods:
a) Either do it like most of the people including me do - marinate the karela with generous amount of salt and keep it aside for 15-30 mins. squeeze them after 15-30 mins nicely (I even wash it once as I really can not stand its bitter flavor at all in the curry) and use it for the curry. The bitter salty water can be taken as a shot alone consoling yourself that you've not wasted anything and you are getting its health benefits :P
b)If you are in a hurry or do not like the idea of marinating in the salt, you can make the way my MIL makes.Before slicing or nicking the karela, make sure you have done every other preparation. As the bitterness of a Karela enhances once its cut (shocked when I came across this tip but this is really a good one). So make sure u immediately put the karela in oil after cutting.
The first method makes Karela absolutely without bitterness, The second one if you do correctly makes one with just a hint of bitter flavor.
3) another tip - if you realize that your karela still came out bitter, make it sweet and sour way- add some tamarind paste and some jaggery (or sugar) after they are cooked. heat for a minute. and thats it. They taste fantastic and totally cover the bitter flavor!
And Thanks for the vinegar and chinese gourd tip
can't wait to try.