don't say ummm at all...
really, it's so annoying, it makes it sound like you don't know what you're talking about.
if I ask you a question, and you say, ummmm, perhaps this is the answer, or ummm, I don't know, or you say ummm,, whilst you think. then that's OK.
but if you're giving a presentation you should really have practised the whole thing start to finish, you should know how long it is, you shouldn't be stumbling over your cue cards or your slides, it should be slick and smooth.
honestly, I'd rather hear nothing whilst a slide has been dropped or something hasn't gone quite right than I would hear ummm. it sounds so unprofessional, like you couldn't be bothered to prepare properly.
other advice, talk sternly, but don't shout. what I mean is talk authoritatively.
if you can, give the presentation to a different audience before hand, and have them ask you questions.
write their questions down if you need to and prepare answers, -that way when you say any questions at the end you'll know the kind of things people will ask.
by presenting to others before hand you'll get a chance to see what your presentation and presenting skills are like, is your presentation boring, does your audience look bored. is it too advanced, does it ramble on and on without coming to a point?
basically, when you present for real you'll need to have a clear, concise and to the point presentation, you'll be expecting what question may be asked and have some idea in your head about how you'll answer them.
if you're presenting with more than one person then it's twice as important to practise, if you're presenting with three, I'd say three times as important to practise, your handover to your colleagues for the next bit needs to be smooth.
not
person 1: "soooo.... ummm...., yeah, so that's about it for my bit, over to person 2 now".
person 2: "yeah..., so... umm..., for bit bit I'm going to talk about something, but I'm not quite clear what yet because I was so wrapped up in not paying attention to him that I didn't really think it was my bit coming just yet, oh yeah, that's what I was going to talk about, I've just got to sort out my cue cards... we did this already. I must have forgotten to change the card."
rather than that, your hand overs should be.
person 1: ...finishes their part of the presentation... "thankyou for listening, now I'm going to hand over to person 2 who is going to explain the inner workings of xyz".
person 2: thanks person 1, "good morning, I'm going to talk about, xyz, xyz works by..."
you may think that sounds rambling, but it's not... it's presented clearly and efficiently, clearly marking the end of your section and combining it with an introduction to the next speaker.
your presentation should read like a well formed technical book, where at the end of each chapter it says read on to learn about xyz, then there is a title marked xyz (which is physically said aloud in your presentation), then goes on to explain/talk about xyz.
know what you;re talking about and talk authoritatively, but when it comes to the audience asking questions, don't act like a know it all.
don't talk with disgust if they ask a pretty basic question.
Remember there are no stupid questions, if people start asking really basic questions, then there is a strong chance that your presentation is at fault, you may have missed something, or be presenting at a technical level that's too high...
if it's a long presentation, you may think about dividing it into sections and allowing people to ask questions midway through your presentation...
if you do this, be concious of the time though, don't let your 5 minutes Q&A about the first section eat into the time that you're planning to present the second section in, cause then you'll have to rush the second section, you'll be on the back foot, and the presentation will suffer...
and if you do have midpoint Q&A sections your hand overs should be.
"thankyou for listening, Now I'd like to ask if anyone has any questions about the presentation so far..."
Take questions, then when your five minutes is up, even if there are more questions
"thankyou, that is unfortunately all the time that we have for questions at this point, now I'm going to hand over to person 2 who is going to explain the inner workings of xyz".
if you have a question in your Q&A that would be better handled by a different person in your team, introduce them in the same fashion as the hand over,
person 1: "I'm going to defer your questions to person 2"
person 2: "thankyou person 1, ... then go on to answer the question"
Honestly, if there is more than one person script the hand overs,
and actually practise your presentation aloud, not just in your head. you read faster in your head than you can talk.
also practise aloud so that you can be sure that you can actually pronounce all the words smoothly and that there is nothing in you presentation that will leave you a bit tongue tied.
Nah just kidding on that part 
