When you shake hands (make Musafaha) you should make the following dua:

يغفر الله لنا و لكم

Rasulullah (sallallahu alaihi wasallam) said that when two believers shake hands, then before their hands part, all their (minor) sins are forgiven.

When you make Takbeer-e-Oola (i.e. saying Allahu Akbar when commencing Salaat) it is FARDH to verbally say the Takbeer, such that at least you can hear yourself. If you merely say it ‘in your mind’ and do not (at least) move your lips, your Salaat is NOT valid!!

It is NOT permissible for two persons to whisper to each other when there is a third person present. The same applies to two people speaking in a foreign language when a third is present and he/she does not understand that language.