The Kitzur summarizes the rules of standing in prayer, including the basic posture of the Amidah and the need for solemn concentration.
יְכַוֵּן רַגְלָיו זוֹ אֵצֶל זוֹ, כְּאִלּוּ אֵינָן אֶלָּא אַחַת, לְהִדָּמוֹת לַמַּלְאָכִים שֶׁנֶּאֱמַר וְרַגְלֵיהֶם רֶגֶל יְשָׁרָה, כְּלוֹמַר רַגְלֵיהֶם נִרְאוֹת כְּרֶגֶל אַחַת וְיָכוֹף רֹאשׁוֹ מְעַט לְמַטָּה, וְיִסְגֹּר עֵינָיו שֶׁלֹּא יִסְתַּכֵּל בְּשׁוּם דָּבָר, וְאִם מִתְפַּלֵּל מִתּוֹךְ סִדּוּר לֹא יָסִיר עֵינָיו מִן הַסִּדּוּר, וְיַנִּיחַ יָדָיו עַל לִבּוֹ, יְמִינוֹ עַל שְׂמֹאלוֹ וְיִתְפַּלֵּל בְּלֵב שָׁלֵם, בְּאֵימָה וּבְיִרְאָה וּבְהַכְנָעָה, כְּעָנִי הָעוֹמֵד בַּפֶּתַח, וְיוֹצִיא אֵת הַמִּלּוֹת מִפִּיו בְּכַוָּנָה, וּבְדִקְדּוּק, וְיִתְפַּלֵּל כָּל אֶחָד כְּפִי הַנֻּסְחָא שֶׁלּוֹ הֵן אַשְׁכְּנַז הֵן סְפָרַד, וְכַדּוֹמֶה כֻּלָּם יְסוֹדָתָם בְּהַרְרֵי קֹדֶשׁ, אֲבָל אַל יְעָרֵב תֵּבוֹת מְנֻּסְחָא לְנֻסְחָא כִּי כָּל נֻסְחָא תֵּבוֹתֶיהָ מְנוּיוֹת וּסְפוּרוֹת, עַל פִּי סוֹדוֹת גְּדוֹלִים, וְאֵין לְהוֹסִיף אוֹ לִגְרֹעַ. (ס"ח צ"א צ"ה צ"ח). לֹא יִסְמֹךְ עַצְמוֹ עַל שׁוּם דָּבָר, אֲפִלּוּ סְמִיכָה כָּל דְּהִיא. וְאִם הוּא חוֹלֶה קְצָת, יָכוֹל לְהִתְפַּלֵּל אֲפִלּוּ יוֹשֵׁב אוֹ שׁוֹכֵב, וְהוּא שֶׁיָּכוֹל לְכַוֵּן דַּעְתּוֹ. וְאִם אִי אֶפְשָׁר לוֹ לְהִתְפַּלֵּל בְּפִיו, מִכָּל מָקוֹם יְהַרְהֵר בְּלִבּוֹ. (צ"ד).
Your feet should be placed next to each other as if they were only one, to resemble the angels, of whom it is said: ''And their feet were straight feet." Which means their feet [ragleihem] appeared as one foot [regel]. Your head should be bent slightly, and your eyes closed, so as not to gaze at anything. If you are praying from a Siddur do not remove your eyes from the Siddur. Your hands should be upon your heart, the right hand upon the left. You should pray wholeheartedly with fear, reverence and humility, as a poor man standing at the door; and pronounce the words with concentration and exactness. Everyone should pray according to his [accustomed] form [of prayer], whether it be Ashkenazic or Sefardic, or another, [for] they are all based on sacred origin. However do not intermingle words from one form with another, because the words of each form are numbered and counted, based on important mystical concepts, and nothing should be added or subtracted from them. You should not lean on anything [for] even the slightest support [is forbidden]. If you are slightly ill you may pray even while sitting or lying down, provided that you are able to concentrate. If you are unable to utter the words you should meditate in your heart.