Athene Donald recently addressed a big pet peeve of mine: conference talks that run way past their allotted time, thus screwing up the schedule for everyone else who has the misfortune to be after them.
When I've presented at conferences, I've usually had dire warnings to keep to the time limit, or else. And I respect those limits. How do I do that?
I practice with the material until I have a good sense of the "beats" of the presentation and I'm reasonably sure that I can end within a few minutes of the target. I also have a couple of ideas about where to expand in case I race through/forget something (nerves on stage) and it looks like I'll have some extra time at the end.
I give the presentation to interested parties internally as a trial run. In grad school and at work, I could always find someone with a stopwatch and a willingness to rip into the slides, the format/order of what I'm saying, and any bad speaking/presenting habits (Have a death grip on the podium? Hem and haw? Accelerate madly as the talk goes on?). But the best way to simulate a conference talk is to collect a reasonably large group of marginally interested people (students/staff who are just there for the promise of food/extra credit/a break from working) and present to them.
When I'm actually up there, I don't rely on a room clock (although it's nice to have one). I have a watch that I can strategically stick somewhere on the podium. And I work out a warning schedule with the moderator, if they haven't already established one. By the time I'm actually ready to present, the watch/moderator are really there for peace of mind and to be able to add a little more info to fill in a minute or so as needed.
So after I've done all my prep work, I find it incredibly annoying to sit through someone else's presentation that has clearly never been practiced and/or could never fit into the allotted time. For example, they spool up a power point with 100 slides for a 20-minute presentation, and the first slide is a wall of text. That's just disrespectful to the audience.
At the same time, it's really hard to provide context and say something interesting in a very short timeframe. I think that conference organizers are shooting themselves in the foot if they have to enforce a time limit of 15 minutes or less per presentation, unless there's some mechanism for speakers or their supplementary material to be immediately available to discuss/expand on/clarify things.