Summarize your essay for the least expert member of your target audience
In the last "Tip of the Week" I suggested that you write a one page (or less) essay describing all the points you plan to make in the presentation. Now it's time to see if you can describe that essay verbally, without slides. One preliminary task is to think through your target audience, and in particular, the least expert member of the audience you hope to reach (not everyone, but the least expert person you hope to reach).
Start by giving a one or two minute version to the person at the next lab bench. You will be surprised at how much he/she doesn't understand the first time you give it. Work that out and try it on someone at the lab next door. Keep working your way "down" toward the member of your target audience with the least knowledge of your material. If you want your talk to be understood by a recent biology graduate then you don't need to practice on an English major. However limiting your practice to a postdoc won't help you reach that recent graduate.
This approach will help you deal with the variation within individual audiences. Summarizing your essay for the least expert person will enable you to create a simple story and narrative, even if you add detail that only the true expert can understand.