Personally, I think that segulos are psychological aids. It's not that you bake a key in your challah and magically you somehow make more money. Rather, the baking of the key in the challah is supposed to be a symbolic gesture that will convey to you that all parnassah really comes from God. And once that becomes more clear to you, it will help your prayers, and your trust in God, and hence your observance of the mitzvos, which actually is a valid form of gaining this-worldly benefits (see Deut. 11:13).
Now, I will compare this concept to two other phenomena:
- Tzitzis. There is a commandment to wear tzitzis on every four-cornered garment with one string of techeles. Why? "וּרְאִיתֶם אֹתוֹ וּזְכַרְתֶּם אֶת-כָּל-מִצְוֹת יְהוָה, וַעֲשִׂיתֶם אֹתָם" - "And you will see it and you will remember all the commandments of God, and you will do them". Somehow, wearing these strings will provide a reminder of God's commandments. They are somehow symbolic. They are a psychological aid. I like to think of segulos as extra-biblical tzitzis.
- Symbolism of the Prophets. We find this all the time. In order for a prophet to convey a certain message to the people, God would tell them to do something symbolic to help the people understand the message. Isaiah walked around naked and barefoot for days (or maybe years). Jeremiah harnessed himself to a yolk. Ezekiel laid on his side for hundreds of days, and then turned around onto his other side for a few more. Hoshea married a prostitute. Etc., etc. You get the point. Why did they do these things? Why didn't they just tell the people God's message? The answer is obvious. Sometimes you can read something or have something told to you over and over again. But when someone acts it out, when it happens in practice, that's when it really sinks in. Same here. You can read about God's hashgacha and hear lectures about it all day. But the symbolic gestures, when you do something active to show it, sometimes that's what it takes to sink in.
The consequence of this is that segulos only "work" if they are meaningful to you. If you do a segula because you lost a bet, or with skepticism or sarcasm, you're missing the point. You do a segula because you grasp the symbolism. If it's something that turns you off rather than being at all meaningful, it's counter-productive.
I'm not going to claim that this is how everyone has ever understood segulos. There will probably be statements across the vast ocean of Jewish literature that contradict this. Chida (for example) may not have understood segulos like this. But I think that this is a valid approach to them, and perhaps how many people have understood it.