Try writing out what you need to say for your presentation. If you know what you're going to say before hand, it reduces the spontaneity of speech. Next break up the words into syllables and practice saying each syllable at an constant pace. You can use a metronome for this. Starts at 90 beats per minute or 60 if you find it too fast and work your way up to 120. Remember to take a breath and make sure you're never out of breath. Practice this for at least 15 mins a day until your presentation. Maybe even twice a day, morning and before bed. What this does is that it helps train your speech and breathing to stay at a constant rate. As stutterers speaking at a constant rate or singing induces fleuncy and makes speaking less block prone. The more structured your speech is, the easier time you have saying it.