What are some good practices when asking people to share their PowerPoint presentations from a conference/talk?
There is no magic when it comes to asking for presentations. And there is nothing wrong with being the person who asks people for their powerpoint slides (at least you show that are interested and it may result in them being cited; and it shows that they got their job done - though a presentation they interested others in their idea).

If they don't reply (or don't sent it), it is usually of one of the following reasons:


They are busy and missed your mail,
It would take their time to find the presentation and send it (it may be big so it is not just sending an e-mail),
They would prefer not to make it public as:


It is not polished enough for anything but a presentation,
It may contain things that they would prefer not to share publicly (e.g. plots form other papers, preliminary data which may later proven to be wrong or incomplete, pictures or video they don't have right to share further, etc).



If they don't want to share it - they have right to it. However, usually they have nothing against (and actually are happy to do so) as long as you make it quick and easy for them.

So it is a good idea to ask for slides just after their talk - they may have it on their computer (so you can copy it to your stick) or on a stick (o you can copy it to your computer) or send it right away.

Also:


Send your e-mail at most a week after the presentation,
Be short and concise (what exactly you want, what do you want with do with it),
If there is no response try writing the same e-mail a week later.