Generally I type with both thumbs. Learning to use a small keypad like on a BlackBerry, or any mobile phone for that matter, is a matter of repetition. You just have to keep using it and get used to the size.
Auto-text is one of the greatest time savers ever. AutoText
Keyboard shortcuts like L for calendar, D for notes, etc, are also tremendous aides for getting the most out of your BlackBerry. http://docs.blackberry.com/en/smartp...s_38809_11.jsp
Additional tips can be found at http://www.stinsonddog.com/
and also http://www.blackberryfaq.com/index.php/BlackBerry_FAQ