For academic conferences in particular (I'm in philosophy, things may vary in other fields), I would add some (perhaps more cynical) advice.
For instance, it is true that the main goal of a presentation at a conference is to test and improve ideas before sending them to publication, by showing them to a specialized audience. But, congresses are also (very importantly) about networking and PR, and making yourself known in the field. A conference brings together specialists in a subject, who will be future editors, reviewers, and professional contacts in general.
I tell my students things like:
- Whenever you can, ask (pertinent) questions. This is important beyond helping the speaker, it is also a way of advancing your own career.
- Think about possible questions while listening to the presentation instead of waiting until the talk is over to start thinking about what to ask.
- If possible, raise your hand as soon as the q&a session begins. At some conference sessions there is little time for questions, and some members of the audience who want to may not get to ask.
- Even when that is not the case, you can help avoid a moment of silence right after the talk has ended, which (as a speaker) can be uncomfortable. The worst outcome for a speaker is for nobody to engage.
- It is useful for the speaker to hear many different questions and points of view. Do not give parallel presentations in the questions. Questions should be concise and to the point. Formulating your question should not take more than a minute.