The Gemara in Masechet Keritut establishes that "Simana Milta," meaning, the things we do as a promising "Siman" ("sign" or "omen") on Rosh Hashanah are significant and have an impact upon the coming year.
Therefore, one should not treat lightly the custom to partake of certain foods on the night of Rosh Hashanah as a good Siman, such as gourd, apples, pomegranates, and so on. If a person does not enjoy one of these foods, it suffices for him to simply point to the food and recite the "Yehi Ratzon" prayer. It should also be noted that one may come up with his own ideas for foods as Simanim. For example, it is told that somebody would eat on Rosh Hashanah a combination of lettuce, raisins and celery, and declare, "Let us have a raise-in-salary."
The concept of Simanim extends beyond the traditional foods eaten on the night of Rosh Hashanah. Although it is important to refrain from anger all year round, one must exercise extreme care not to become angry on Rosh Hashanah, since one's conduct on Rosh Hashanah sets the tone and serves as a Siman for the rest of the year. Very often, the length and intensity of the synagogue services can cause a person to become edgy and irritable. One must therefore be particularly careful to avoid anger during this holiday.
The Talmud Yerushalmi famously remarks that if one sleeps on Rosh Hashanah, his "Mazal" (fortune) for that year sleeps, as well. This does not refer to sleeping during the nighttime hours, but rather to sleeping during the day. One should try to rise at daybreak on the morning of Rosh Hashanah so that he does not sleep at all during the daytime hours. The Arizal permitted sleeping after midday if a person feels tired, but one should make a point of remaining awake from daybreak on Rosh Hashanah, until at least midday.
The Halachic authorities note that "sleeping" for purposes of this Halacha refers not only to actual sleep, but also to wasting time, such as engaging in idle chatter. One who remains awake on Rosh Hashanah but does not spend his time productively is included under the Yerushalmi's warning that his "Mazal" will sleep, Heaven forbid. Therefore, it behooves one to not only make a point not to sleep during the day of Rosh Hashanah, but also use his time productively and meaningfully.
Summary: One should ensure to partake of the foods traditionally eaten on Rosh Hashanah as a favorable "sign" for the coming year; if one does not like one of these foods, he can point to it and recite the "Yehi Ratzon" prayer. One must ensure not to become angry on Rosh Hashanah, and one should try to rise at daybreak and remain awake until at least midday. Additionally, one should use his time meaningfully on Rosh Hashanah, and not waste his time with idle chatter and the like.
