Never say these things when giving a talk....

1) "I don't think I need to say it to THIS audience......". Just don't say it. You're wasting everyone's time, not to mention that you appear patronizing. Or rather, actually you should always say what you were going to say anyway, as you can rest assured that the vast majority in your audience has either never heard of it, or would like to be refreshed before you launch into your Biblical details....

2) "As you probably know....". Same thing as above, probably no one knows, so just SAY IT.

3) "As you can clearly see....." right after putting up a brand new slide. No, no one can clearly see, because you have neither explained the context of the studies nor the exact experiment that was done to obtain the data. Sadly a very common occurrence, particularly in talks by senior scientists......

4) "We were able to show....". Let me tell you how this currently popular sound-byte really sounds like. Though the thing you are showing the audience wasn't really there when you looked the first 1000 times, somehow - after five sleepless nights - the unputdownable postdoc was able to get the data that everyone really wanted to see (and what is on the screen right now). Even if true, this is hardly the best approach. "We saw..." is much better IMO. 

5) "In the interest of time, I'll skip these last 25 slides....". One or two is OK, but when you are flipping through dozens of slides on the gigantic screen, rest assured that someone in the audience is having a seizure. 

.....and the WORST one....

6) "I want to make this seminar informal, so please feel free to ask questions as we go along....". You can rest assured that there is a career "question-asker" in your audience who is going to stop you every 10 seconds to ask a question that everyone else knows the answer to.