Some good advice on the subject is here, here and here (the last two are primarily intended for the mathematicians), here (this one is primarily on giving short talks) and here (this one also contains some helpful links to writing tips). As for the job talks, see e.g. this article by Richard Reis. On a related note, see also his article on getting the most of your conference trips.
Update 1: Presentation Guide for Scientists by Ad Lagendijk
How to Conquer Public Speaking Fear by M. Orman
Update 3: How to Give a Great Presentation at the To Done blog