Dov Bear had a very cynical post regarding the notices and posts yesterday about the segullah of reading Parshas Ha'Man. He questions how people could believe that if they say the specific prayer all of the sudden they will get rich.

On the issue of suddenly getting rich because you read Parshas Ha'Man, he is mistaken. It is not brought down that Parshas Ha'Man is a segullah for wealth (osher) or for money. It is brought down as a segullah for parnassa (for lack of a better definition I will say it means "support"). It could be that you get the raise you asked for in your current job, because you said Parshas Ha'Man. Or maybe you will find a better job, or maybe you have been unemployed and now you have interbiews to go to. Nobody promises instant wealth by saying a segullah.

On the idea of reading Parshas Ha'Man to effect successful parnassah, I would venture to say that the way it works is because of what Parshas Ha'Man is about. Parshas Ha'Man is the story of how the Jews travelled through the desert and had nothing to eat. God decided to support them by providing them with their daily fill of food, via the manna. It was a miraculous event. By reading this story, it causes one to remember that Hashem is our provider and he does it through our efforts.
When a person has a good job and is successful, he can easily fall into a rut thinking he is very accomplished and talented. he thinks he got promoted or given that big fat bonus because he is such a smart guy and so talented, or he performed very admirably in a certain project, etc.. A person can easily forget that it is God who sustains all of us, albeit through seemingly natural systems. By reading Parshas Ha'Man, it gives us an opportunity to focus on what is really happening. It gives us an opportunity to rethink our positions in this world and realize that it is Hashem who is providing us with our daily bread, via the CEO of whatever company he works for.
That is the segullah of reading Parshas Ha'Man (I do not know why specifically on the 9th of Sh'Vat). By focusing on these ideas and realizing Hashem's assistance given to us, we focus our prayers and our actions differently to reflect that. Then we are more deserving of Hashem's assistance.

It is a similar idea to the one behind various mitzvot we perform regularly: to tithing one's produce, not working the fields in the shmittah year, not working on shabbos, etc.. among others. They cause us to remember that Hashem is the real Provider, behind the scenes.

On the issue of segullahs in general and how they work, I spoke to a (very intelligent and knowledgeable) Rav not too long ago about the idea of segullahs. He explained to me that segullahs do work. However, they should be used with caution. He explained to me the way segullahs work is like a tablecloth, for example. let's say your tablecloth is just slightly too short for your table and does not reach your end of the table. You can pull the tablecloth towards you and it will reach your end. However, you must realize that by doing that you have pulled it off the other end.
Segullahs are like that. They do work and by performing the segullah you will (sometimes/often) achieve the subject of the segullah (e.g. parnassah, shidduch, wealth, long life, children, whatever), however you must realize God is giving it to you by taking it off the other end. Meaning, if this is something you may not have had coming to you, God might say ok fine you can have it, but He will deduct it from something else that you were supposed to have.

Segulahs should be used and performed with extreme care and caution. Not frivolously.