Ramsch
Ramsch happens when all three players pass in the auction: nobody wanted to declare, and now nobody wants to win tricks either. Officially that triple pass voids the round; at this table you play Ramsch instead. It's played like Grand, only the four Jacks are trump, and there is no declarer: all three of you play for yourselves. Whoever ends the round holding the most Augen loses that total as minus points, the other two stay at zero, and winning every trick instead, a Durchmarsch, flips the whole round: a flat +120.
Read the official rule on Ramsch scoring and the Durchmarsch.
Ace and Ten of the same suit: twenty-one Augen you would much rather not be the one holding when the last card falls.
Leading
The safe low lead
With nothing more urgent to do, lead a low card (7, 8, or 9) from a suit where you still hold at least two cards. A card that low rarely wins on its own, and keeping a second card in the suit means you're not forced into a riskier lead there next time.
Leading where you're not on top
Lead a suit where your card isn't the highest one still in play. Someone else is holding something higher there, so your own lead is unlikely to win the trick for you.
Leading to get void
Lead from your shortest non-trump suit, one or two cards, to empty it out entirely. Once a suit is gone from your hand, any later trick led there becomes a free chance to dump your worst remaining card instead of following honestly.
Shedding a low Jack
Lead your lowest Jack while a higher one is still out there, unplayed. Once every higher Jack has fallen, whatever Jack you still hold becomes the highest trump left and wins the trick outright, Augen included, so get rid of the weak one now, while it still has company above it.
When nothing else fits, lead low
None of the above quite applies? Lead your lowest non-trump card, or your lowest Jack if trump is all you have left. It's the safest default: no clever setup, just the smallest chance of winning a trick you didn't want.
Following: ducking and dumping
Void in the suit led? Dump your priciest
Out of the suit that got led, you're free to play anything. Throw your highest-point card among whatever non-trump you're still holding; trump only comes into it once you have nothing else left to throw.
Last to play: dump the priciest loser
By the time a trick reaches you last, you can see exactly what beats what. If a card in your hand can stay under what's already on the table, play the most expensive one that still does, and get those Augen off your hand for free.
Already the biggest loser? Keep dumping
Second to play and already at least ten Augen worse off than both of the other two, waiting to see the third card is a luxury you can skip. Dump your most expensive loser now: you're already carrying the risk, so there's little left to protect by playing it safe.
Second to play, not yet behind: play it safe
Second to play and not yet the one in trouble, hold back instead. Play your cheapest loser rather than your costliest: the third player still has to act, and you'd rather see what they do before committing a valuable card to a trick you might not even need to lose.
Nothing stays under: win as cheaply as you can
Sometimes every legal card in your hand beats what's already on the table. There's no ducking out of that trick, so take it with the cheapest winner you have and keep your bigger cards for later.
Otherwise, just play low
When none of the above quite fits, fall back on the simplest rule there is: play your least valuable card. It costs the least if the trick ends up going the wrong way.
The endgame
The last trick decides the Skat
By the tenth trick, everyone's down to exactly one card, so there's no decision left to make when it's led: you play whatever's still in your hand. The real work happens two tricks earlier: through tricks eight and nine, shed your higher cards so the one card you're left holding for the last trick is your lowest, the one least likely to win it. Win it anyway and you don't just take its points: the Skat's two hidden cards go with it, Augen included.
Going for the Durchmarsch
Win five tricks in a row and Lothar switches into hunting mode, confident enough in the run to chase all ten instead of just avoiding points along the way. The Durchmarsch itself only pays out on winning all ten, a flat +120. From there, keep leading your strongest card, trump first, to press the sweep through to the end; following instead, take the trick with your strongest winner when one is legal, or throw your cheapest card when nothing you hold would win it.
Stopping someone else's Durchmarsch
If an opponent has swept every trick so far, five or more, breaking that run outweighs everything else, even the last-trick caution above. On lead, play your strongest card, trump first, to take the trick outright and end the sweep. Following, win as cheaply as you can if a winning card is legal, or throw your cheapest card when nothing you hold would win it; void in the suit led, play low instead rather than hand over any more Augen. The points a Durchmarsch would cost you dwarf whatever the Skat was ever worth.