The Ramban says that a decision made in Heaven, may not necessarily be carried out and can, in fact, be changed. If we make an action here on earth which resembles that decree, it will facilitate the fulfilling of that decision.
Hakham Yosef Hayyim, 'a"h, writes that the custom of opening one's hands when reciting the verse "Potheyah Eth Yadekha" (open your hands), which is connected with one's Parnassah (livelihood), is a good practice, because it symbolizes the opening of one's hands to receive something. This, in turn, helps facilitate the receiving of the blessings from above. This is one of many examples which illustrates the Ramban's statement.
Let us explore one of the simanim of Rosh Hashana, the pomegranate. When we eat a pomegranate on Rosh Hashanah and recite "May it be Your will that we will be filled with a good deeds like [the seeds of] the pomegranate", we prepare the ground for ourselves to indeed be loaded to capacity with good deeds. Undoubtedly, a mere act of saying and eating is not sufficient for turning us into such people, as we are required to improve and work hard to collect good deeds and perform the Miswoth (commandments). However, we should not belittle the importance of those "Simanim".
All year round, and particularly on Rosh Hashanah, our acts carry weight with regard to our future. Since Rosh Hashanah is the day that Adam was created, it is the root and source of our existence, and therefore our acts on this day are most significant. When our Hakhamim warn us specifically not to get angry on Rosh Hashanah, or to increase the amount of sweet foods we eat, they teach us the importance of these acts and how they can make a difference for the upcoming year.
From the bottom of my heart I would like to wish every single one of you, a year of sweetness, true happiness, health and growth in our connection with our Father, our King.