First off, carry it with you everywhere. People may laugh at me for being so attached to my timer but that's only because they don't truly understand why it is there. I always have mine clipped to the pocket of my pants. This works well except for when I bump up against a counter or something and end up knocking it off onto the floor. I have gone through a number of timers due to this. Timex should really get into making lab timers. The ones I have can't take a licking and keep on ticking unfortunately. Once people see that your timer is literally attached to you at the hip they will just come to accept it and not think anything of it. This is also key. That way when it beeps they will truly believe that you have to get back to an experiment but you will know better
Here's how it works, you need to go ask you supervisor a question or want to show them a result. You know though that as soon as you step in their office they will ask you about all the experiments that you have yet to do. So you think, hmm... how long do I want to be in there for? Five minutes is usually good. So you set your timer and off you go to talk to your supervisor for exactly five minutes. When your timer beeps you say, "Oh, gotta get back to my experiment"
As for that annoying grad student or inquisitive undergrad, if they approach you unexpectedly and you don't have your timer set don't worry the timer trick will still work. You need to be a bit more clever though. Shortly after they start talking glance at your timer. Then say, "Oh shit. I forgot to set my timer, one sec". Then you set it for however long you want to talk to them for. Again, when it beeps excuse yourself with, "Oh sorry, my experiment beckons". You just have to hope they won't follow you and keep talking. If that happens, you're on your own. The timer trick can only go so far.
Now that you know the timer trick use it wisely. Over abuse can result in people getting suspicious. And if you are ever talking to me and my timer beeps I really do have to get back to an experiment :)