There are some systems where one side knows what he's going to play, more or less regardless of what's happening on the other side of the board. Usually it's White who has this luxury, and usually he's playing for some sort of automatic kingside attack. You know the sort of thing - a King's Indian Attack, or Grand Prix Attack, or one of the Colle / London / Stonewall type setups.
It can be rather annoying to play against this stuff. Not that it's objectively so frightening; but practically speaking White is likely to get his attack, and probably also an advantage on the clock in a type of position that he plays every week - and that's not to be sniffed at.
Nevertheless, my approach in defending against all this is generally to allow, or even encourage, White to do what he wants to do. I tell myself that so long as I haven't made any serious mistakes then there's no reason that his attack should deserve to succeed; and that if and when it fails then I have every chance of being better. This seems to me to be the principled approach: I don't believe that you can expect to play automatic moves and checkmate me, and to prove the point I'm going to let you try.
Sometimes, this goes wrong.
Here I have willingly accepted a pawn, and if I find 18. ... Re8 then I ought to be able to hold on to it. But instead, after 18. ... Qd7? 19. Bxf6 h5 20. Bxg7:
... I was still labouring under the misapprehension that I should be winning. Therefore I rejected 20. ... Kxg7 21. Qc3+ f6 22. Rxf6 Rxf6 23. Qxf6+ Kh7 with equality, and instead played 20. ... hxg4? - completely missing 21. Bf6! and losing quickly.
Yes, being principled, and even being right, is all very well; but you still have to play good moves.