If one desires to accustom himself to say vsein tal umatar by saying it 90x (so that he shall not have to repeat his prayer in a case of doubt), may he say it on Shabbos, or is it considered preparation for a weekday?
HE may say it on Shabbos and according to most poskim it is not considered hachana. There are a few reasons given. One is that there is no hachana if the only thing done is speech, unless the words were accompanied by an action, such as saying I am going to sleep so I can stay up tonight, but plain speech is not considered hachana. Another reason given is because it is not considered preparing for tomorrow, rather perfecting oneself to have the correct habit so SE won’t have to be repeated.
Tzitz Eliezer 19-14 (2), Meohr Hashabbos2 pg 596 in the name of R’ S. Z. Auerbach zt”l