If there are too many ideas in your work to present completely in 30 minutes, then pick one or two (the most interesting/important parts) that you will discuss in detail, and present the other parts at a higher level. Also, you can create back-up slides for specific details that you don't plan to talk about, but may get questions about.
This is particularly important for figures and graphs. For example:
You should assume that there will be about 5-10 minutes worth of questions during or after your talk. If your talk is too long, you should cut out some material to get it to fit into the time slot (your audience will not mind if your talk ends 5 minutes early, but they will mind if it goes 5 minutes over).
As a practice talk audience member, you should jot down notes of places in the talk where you have suggestions for improvements, or where something seems unclear. After the presenter is done with his/her practice talk, you should talk through the things you wrote down. It is also good to give the presenter some practice answering audience questions.