While reciting the Amidah, one may not hold an object that he fears will fall, such as tefillin, a book, a full bowl, a knife, coins, or food, because he will worry that it may drop, and thus his kavanah will be disrupted (Shulchan Aruch 96:1). Even in other parts of the prayer service, like Shema and Pesukei d’Zimrah, one must be careful about this. L’chatchilah, one should not hold anything in his hand while reciting the Amidah, for it is not respectful to stand in front of Hashem while holding something extraneous (see Mishnah Berurah 96:1 and 5, based on Talmidei Rabbeinu Yonah, Taz).
Nonetheless, holding a lulav on Sukkot is permitted because it is a mitzvah to do so and it does not disrupt the kavanah in one’s prayer. Similarly, one is permitted to hold a siddur because it is necessary for prayer (Shulchan Aruch 96:1-2).
L’chatchilah, one should not recite the Amidah while standing with a knapsack on his back, for that is not a respectful way to appear before important people, and all the more so, it is not respectful to pray in that manner. However, if he is already traveling with a knapsack on his shoulders and it is more comfortable to leave it on, he may pray with it on him if it weighs less than four kabin (approximately 5.5 kilograms or 12 lbs, 1.5 oz). If the knapsack is heavier than four kabin, he is prohibited from praying while wearing it because such a load is liable to impair his kavanah (Shulchan Aruch 97:4).
Additionally, if someone holding tefillin or money fears that if he puts these items down they will be stolen, and he does not have a friend there to watch them, nor pockets in which to put them, it is preferable, b’dieved, to keep them in his hands while praying, so that he will be less troubled (Mishnah Berurah 96:6; Kaf HaChaim 7). Likewise, if someone carrying a heavy knapsack on his back is worried that it will be stolen, and he has no other choice than to carry it, he is permitted to pray while wearing it.
A soldier carrying a gun, l’chatchilah should not pray with his weapon on him, nor enter the synagogue with it, for it is inappropriate for him to pray about life and peace while wearing an instrument intended for killing. However, he may pray with it on him if he is carrying it for security reasons or guarding it from theft. If possible, he should take the magazine out of the gun, so that it will be considered less of a weapon. When, for security reasons, it is best that the gun be loaded, he is permitted to pray with the magazine inside (see Tzitz Eliezer 10:8).
A person who has a cold must wipe his nose before praying so that he needn’t do so during the prayer service. If phlegm in his throat bothers him, he should expel it before praying so that it will not distract him during the prayer service (Shulchan Aruch 92:3). If he must wipe his nose while praying, he should do so in the politest way possible. Similarly, if he needs to yawn, he should cover his mouth with his hand. This is because a person who stands in prayer must be very careful to show respect for Heaven, and all actions that are considered impolite before people are also prohibited during prayer (see Shulchan Aruch 97:1-2).