Tips on Talks
The usual rules apply:

Get the physics right.
Face your audience.
Speak loudly (enough) and clearly.
Make sure your written material is spelled correctly.
Keep your storyline clear.
Make sure your audience is following your story.
Use reliable sources, and verify any calculations yourself.

Because these talks are longer and more complex than the introductory physics problems you've done on the board, there are more rules:

The basic design is, "Tell 'em what you're gonna tell 'em; tell 'em; tell 'em what you told 'em." Your talk's title and your outline slide ought to make it clear what your subject is. You generally need an introductory portion, a body, and a summary.

It's also very important to tell your audience, at the beginning, why they're listening to you. Your subject is of great scientific or sociological importance, or it's very beautiful, or something.

Your outline slide can reappear throughout your talk, so that your audience is reassured that you are progressing to a happy ending.

At the end of your talk, besides summarizing the material you've presented, you may want to talk about the future, or about implications or aspects of your material that you haven't covered in this talk.

Also at the end of your talk, acknowledge those who have worked with you and display references to any sources you did not credit during your talk.

Your last words should be "Thank you," so that people know it's time to applaud.

Power point presentations are subject to some special rules:

Never overdo the effects, as this causes some people to think you're trying to distract them from a lack of substance.
Make sure you have a strong contrast between your text and your background color or image. Verify this on a screen, as contrast is often stronger on your computer display.
Don't put too many equations, or even words, on one slide.

Other considerations:
Demonstrations can be fun and convincing.
Practice your talk so that it's the right length and your transitions are smooth.
If your slides are done well, all you need to do is to describe each slide to your audience.
Any time you show a graph, clearly identify the vertical and horizontal axes.
If you're doing a calculation on the board, get it right but don't do it with your nose buried in your notes. Practice makes perfect.