Practice your presentation with the clock counting up.
As you’re cutting material, rehearse with a clock counting up, not with a timer counting down. If you go over, you need to know how much you’re over. Give critical content the most stage time. Then, cut sections that are *more* important to you than to the audience. Keep trimming and practicing until you’re consistently within your desired timeframe.
Practice with a timer counting down.
Once you’re within the timeframe, begin practicing with a timer counting down. Divide your content into quarters and calculate a time stamp for the end of each quarter. For example, if you’re giving a 40-minute talk, know the exact slide you should be on at the 10-, the 20-, and the 30-minute marks so you can gauge throughout the talk if you’re on time or running over. That way you can trim more easily on the fly.