[PhatTBoll]

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To make a good steak:

Don't cook it on one side the flips it and cook it on the other side. Flip it as often as possible. If you only flip it once then the juice rise to the top then you flip it and they all fall off. If you flip it often then the juices can't escape and you will have a very juicy steak.

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Repeatedly flipping a steak will lose you far more juices than just leaving it alone. But, as long as you refrain from cooking the hell out of your steak, you should have enough juices left in it regardless of how many times you flip it.
One more thing: you should indeed sear both sides of a steak before letting it cook, but it's not to "keep the juices in." It's to caramelize the surface, which gives you the flavor combinations that you associate with a really good steak. This is called the Maillard reaction.