Are there any specific halachot about one who practices reciting prayers? Is one allowed to say Hashem's name? What if one does not use any tune when practicing the prayers? What if while reading the prayers, one begins to feel emotion and actually pray (at the wrong time, such as with Rosh Hashanah prayers before the holiday)?
1 Answer
The Maateh Ephraim (581:58) recommends practicing or familiarizing yourself with the Rosh Hashanah prayers beforehand;
ראוי לכל אדם לתת לב קודם ראש השנה, לסדר לעצמו התפילות והפיוטים, וילמוד פרוש הדברים, שיהיו שגורים בפיו בראש השנה. גם ילמוד קודם ראש השנה לבניו הקטנים ובני ביתו סדר תפילות, כדי שלא יצטרך להפסיק בראש השנה להראות להם הסדר:
If your saying Hashem’s name in a passuk then it should be no worse than if you would read the passuk to practice leining or something. As long as you don’t stop in middle of the passuk then it should be ok. If it’s the Name in a bracha then its a bracha l’vatala and is not allowed.
Getting carried away and praying an extra prayer is technically allowed as a tefillas nedava but requires special concentration and intent. The Chayei Adam and others strongly discourage it. See https://www.etzion.org.il/en/halakha/orach-chaim/prayer-and-blessings/tefillat-nedava-voluntary-prayer For a full discussion of the issue of offering extra voluntary tefillos.