A good few large drinks beforehand is sensible to celebrate the end of being a civvy. 7 or 8 pints, or double nips should suffice.
If the person administering your oath gets lippy, say "I am ready to serve my Queen and Country, and defend them from Johnny Foreigner and his evil ways. I won't end up as a Passed Over Major like you."
That will show them that you mean business, and you will not get messed around by the REMFS during Basic.