Offense vs. Defense

Everyone in chess wants to win.
Everyone wants to attack.
Well, I am no exception... I am recently back in the chess world, and have been playing one or two games at a time, so that what time I do devote to chess, I can spend analyzing my games more in depth, so to give myself a good chance of success. Now, I enjoy attacking as said, so coming back, I have been choosing the Blackmar-Diemer as white when possible, even in fast games, just to eliminate the Queen's Gambit lines that everyone has memorized to the moon and back. I recently played a game, against an opponent, over the board, against a player that was rated over 1700, and true to form, I pulled out the same opening. What transpired got me wondering a question I am going to pose after I show the game.

This isn't the only game I played against this opponent. Our other game, when he was white, was completely different. He played a solid opening, and got his pieces into good positions. So my question is this...
How do you prepare for games that you have never seen? He played a number of the right moves, that I have seen over and over again. And he was even able to get the dark square bishop moving and castle, which is often a problem against this opening. I have fallen a number of times for tricky openings myself, but long analysis can help me navigate them decently for the most part.
But say a person comes up with an opening that has never been seen (highly unlikely at this point but yeah) How do people suggest preparing for something like that? Do you endeavor to just transpire every game back into lines you are familiar with? Do you try and lock a position down so you can play positionally? What do people think is the most effective method of safely navigating unknown territory?

Thanks for all advice on this topic in advance!

Peace, Love, and a rubber glove