Outlook Quotefix is a standalone module that ‘fixes’ many of the problems people have with Outlook - specifically the way it handles quoting. Goodbye top-quoting! Goodbye shoddy line-breaks!

From their site:

Outlook-QuoteFix can modify MS Outlook’s message composition windows on-the-fly to allow for correct quoting and to change the appearance of your plain-text replies and forwards in general: move your signature, use compressed indentation, customize your quote header, etc.