The psalm says some trust in chariots and horses. Should I just pray and skip the work?
Start with who the commentators say it was for. Rashi says David sent his army out to fight while he stayed in Jerusalem and prayed for it. The verse he points to is from the war against David's own son Avshalom (Absalom). The people told David not to march with them, because it was better that he help from the city.
So the army went out. Nothing here says it should have stayed home. Rashi also quotes the Gemara: if not for David, Yoav would not have gone to war. The Gemara adds the other side: if not for Yoav, David would not have been busy with Torah. Each one's work made the other's possible.
What the verse does say is where the trust goes. Radak explains that the enemies came with chariots and horses and relied on them, as Yeshayahu 31:1 describes people who "trust in chariots because they are many" and do not look to God. We call on God's name instead, he says, and then even on foot we win. Metzudat David (a line-by-line commentary from the 1700s) adds that our side had little in the way of chariots and horses.
Rashi adds a note on the word "nazkir," usually "we mention." He says it is also the word for burning an offering and for prayer.
None of the commentators quoted here says to put down your weapons. The psalm sorts out what you rely on, not whether you act. How to split the work between effort and trust is something the text does not settle for you.
Rashi on Tehillim 20:2 and 20:8,Shmuel II 18:3,Sanhedrin 49a,Radak and Metzudat David on Tehillim 20:8,Yeshayahu 31:1Is "may you get what your heart wants" a good blessing? What if the heart wants harm?
Verse 5 says: "May He give you what your heart wants and fulfill all your plans." That sounds like a blank check. Verse 6 ends with something close but not the same: "May Hashem fulfill all your requests."
Radak first gives the plain meaning, and it fits a battle. What the heart wants is that God put your enemies under you. The plans are war plans, as in Mishlei 24:6, "with strategy you make war."
Then he brings a midrash. David was whole before his Creator and prayed with a whole heart, so people could say to him, "May He give you what is in your heart." But they do not say that to just anyone, because sometimes what is in a person's heart is to steal, to commit a sin, or to do something that does not fit. To everyone else they say, "May Hashem fulfill all your requests."
Radak tells it about David. In Midrash Tehillim itself, the same lesson comes in a story about Rabban Gamliel (a leader of the sages of the Mishnah), who asked a rabbi named Chalafta ben Karuya to pray for him.
In other words, the midrash treats this wish as one that depends on the person. It does not cancel the wish. It says who it fits.
What the midrash leaves open is how any of us knows whether our own heart is that kind of heart. Radak does not say.
Tehillim 20:5-6,Radak on Tehillim 20:5,Midrash Tehillim 20:5,Mishlei 24:6What if I pray and nothing seems to happen?
The psalm opens with a wish and closes with the same wish: "May Hashem answer you" at the start, "May the King answer us on the day we call" at the end. Midrash Tehillim points out that it begins and ends with answering. But look at the verbs. They are wishes, "may He," not promises.
The same Yerushalmi passage that ties this psalm to the Amidah has a practical line right after: from here they said that someone who prayed and was not answered needs to fast.
The Gemara in Berakhot gives different advice. Rabbi Chama bar Chanina says that if a person sees that he prayed and was not answered, he should pray again. His proof is Tehillim 27:14, "Hope to Hashem. Be strong, let your heart take courage, and hope to Hashem." The verse says "hope" twice.
The midrash adds its picture of the graveyard on the road. It says that when troubles cover Israel, that is when redemption is close.
These sources give different advice: fast, pray again, or keep hoping. All of them assume that prayers are sometimes not answered right away. None of them says to stop.
Tehillim 20:2 and 20:10,Midrash Tehillim 20:2 and 20:6,Talmud Yerushalmi, Berakhot 4:3,Berakhot 32b,Tehillim 27:14