How to deal with castling on opposite sides?

It seems to me that whenever my opponent castles on the opposite side than me (which in the vast majority of cases means that my opponent castles queenside while I castle kingside), I end up losing the game, no matter how I try approaching the situation. If I try to hurry to attack my opponent's king, it always feels like I just lose the race because somehow my king gets attacked first. If I try to add defenses to my king, it rarely helps anything. Just trying to play "normally", without rushing to attack or defend doesn't seem to help much either (because my king gets attacked relentlessly).

What are the principles on how to play when castling on opposite sides?

I guess, broadly, there are two types of attacks, one where you need a pawn break to open some lines, and one where you can just shove 3, 4, or 5 pieces in front of the enemy king, sacrifice to open lines, and checkmate via pure force.

So the first thing I'd hate to see in an opposite side castling game is that 1 or more of the pawns in front of my king have moved. That makes it much easier for pawn storms to open lines, and somewhat easier for piece sacrifices to open my king too.

Umm, there are all sorts of things that can go wrong... including you could be winning for 39 moves and then blunder on move 40. It's hard to give advice if you don't share a game.

I see there was opposite side castling in your recent game vs @tidepod4life -- but that loss had nothing to do with castling. You were lost out of the opening... as early as move 5.

It is a race most of the time.  You have to choose defense or attack, either way stick with your plan.  Any passive move or loss of tempo is doom.  Make every move count!  And be on the look out for sacrifices on both sides to open up the King's defense.

Seriously, like 90% of the time when my opponent castles long, I end up losing. If I castle long, I end up losing. I just don't understand how it works.

I think your problems start with 12.. h6.  Why not 12.. h5 and shut down their coming pawn storm?  Also your next move is maybe worse than that.  Taking the knight opens up the g file knowing he just put his rook there aimed at your king.  If you look at my earlier post I mention not to waste moves.  You lost a lot of tempo that might have been better spent breaking the center or if he managed to castle pushing your own attack.  Your bishops knights make great defenders while your rooks and queen can easily slide over and support your own pawn storm

h5 is only going to be met with white f3 and then g4 in a few moves. There's no stopping the pawn storm.

On that, we agree. But why recommend ... h5? Why make weakening defensive moves when nothing is threatened yet?

Why not threaten to open the center? (eg: play ... Qc7 threatening ... d5) A strong counter-threat in the center will normally paralyze a wing attack. So if you threaten to open the center, White will probably reply by castling Q-side. Then you can play b5, a5 and a4 (replying to bxa4 with b4 and then Rxa4).

Isn't Black's attack faster than White's?

I think you're right.  I was probably trying to better an idea he had and was thinking white could just take more space the next move.  But definately a better strategy is to counter attack in the center, especially since white hadn't  castled yet.

Play on the wings should be met by a central counter, usually by openings lines in the center.

That's the most thematic reply, yes. But a counter-stroke on the opposite flank can also be effective, especially if it is faster than the opponent's attack.

I am not the expert, but in general, you should only move those pawns in front of your king if you have to. There are situations where h4 is met with h5 for instance, but in many cases it is better not to move those pawns and expedite the opponent's attack.

For instance f6 here weakens the king and helps the opponent open lines in front of your king. He should've taken that pawn. That move is playing into their hands. In most of the cases you move those pawns to close lines in front of your king or at least to slow the attack of the opponent. 

In many cases the answer should be creating your own attack either on the other flank or in the center.