Use a good presentation-building tool, like MS PowerPoint. Avoid Latex, except for slides with formulas. Good looks are important. If you need formulas, try TeXPoint, George Necula's Latex for Powerpoint.
Humor is very useful; prepare a couple of puns and jokes beforehand (but not epic jokes, which require complicated setup). However, if you're not good with jokes, better avoid them altogether. Improvising humor is very dangerous.
The more you rehearse the talk, the better it will be. A rehearsal is most useful when carried out loud. 5 rehearsals is a minimum for an important talk.
The more people criticize your talk (during practice), the better it will be; pay attention to criticism, not necessarily to all suggestions, but try to see what and why people misunderstood your ideas.
When using printed slides, avoid overlay slides; they are awkward to use.
Not everything has to be written down; speech can and should complement the information on the slides.
Act your talk: explain, ask rhetorical questions, act surprised, etc.
Give people time to think about the important facts by slowing down, or even stopping for a moment.
Do not go overtime under any circumstance.
Listen to the questions very carefully; many speakers answer different questions than the ones asked.