Most players are not going to think too much about their investigator's Elder Sign (), which comes up so rarely and doesn't really do anything. Seriously, how many effects are just +X or only trigger on particular types of tests? Anyways, if your name is Father Mateo, your whole shtick is about the Elder Sign and getting it as often as possible... but Silas Marsh players should also take note, because this is the strongest one in the game.
First off, the effect -- you get to commit YOUR CHOICE of skill cards from the discard pile, (1) getting its icons, (2) getting its skill card benefits, and (3) keeping it around for another test. Any of the three parts would be good-to-great effects, but all three? Amazing. Let's talk about each: (1) the doesn't give you any skill boost by itself, so in the event you aren't going to pass the test (a test in Mythos, probably), this is almost like a Lucky!. Keeping some icons, like an Unexpected Courage, in the discard pile is key. (2) Oh boy -- we don't just play skill cards for their icons, we mostly play them for their action-less effects. Here, there's NO RISK in committing the card, since you know if you'll pass and trigger the effect. And the big prize here is Resourceful, as long as you're taking a , , or test, and you get to grab ANOTHER from the discard pile. It could be another skill, or a powerful event. But maybe if you're going to pass by 1 (and now 2) anyways, why not give yourself another action with Quick Thinking? Your discard pile is your oyster. (3) If that wasn't enough, you'll get a chance to do the effect AGAIN on a future test. Silas can run low on cards on hand because he commits so many skills, so this solves that problem very quickly.
Putting all three effects together and you get a huge tempo swing. The not so unusual case with Resourceful and Lucky! (3) goes like this: compared to just drawing a 0 or +1 token, you'll pick up two more cards in hand from your discard pile (the aforementioned Resourceful and Lucky!). On a future test that you might fail, you'll commit Resourceful, play Lucky! and get it back, and then draw another card from Lucky!'s effect. You'll never fail a skill test again, and you'll keep replenishing your hand. That's the way to stay ahead of the game.
OK, so how do you make sure this keeps happening? In , it's Eucatastrophe, and Silas / Resourceful is probably why the Taboo list makes you remove the card from the game after you play it. Still, doing it twice per game is plenty powerful. Against All Odds, the only card that potentially reveals multiple tokens, is worth a passing thought, but nothing more than that. For maximum multiplayer fun, though, pair with an investigator who will give you some help. One option is someone who can take Blessing of Isis and will follow you around. Silas Marsh is excellent with tokens as well, so you can really put that Favor of the Sun to good use here. The other option is to get help from a Mystic who will toss you a Seal of the Elder Sign or seal other tokens like with The Chthonian Stone, to make drawing the more likely. Or just play with Jacqueline Fine, who can use her ability on you!
Lastly, the only time that the effect fizzles is when there is nothing in the discard pile you can commit. This is because your discard pile is empty because the game just started -- a strong endorsement for Short Supply -- or you don't have skill cards with matching icons in the discard, like a few icons or Guts in the Mythos phase. That's why it's a good idea to have a variety of skills in Silas and commit when you can. Or maybe you've already pulled everything back from your discard pile with True Survivor or something, in which case you're probably already trouncing the scenario.