Most of the time I do my best to be very productive and keep busy and useful. I keep everything very neat and organized... which takes up a little extra time... but my OCD demands it. I can't count the times my manager has half-jokingly groused at me for straightening her mess to well that she can't find anything.
There are rare instances that I have less than normal to do and it's too close to quitting time to go out searching the sales floor for random tasks; like the last 15 minutes of work. I have found that "Seconds-slacking" can help draw out that last little bit of work to fill in the remaining time.
This keeps me from having ten minutes at the end of my shift with everything done, and enough time for one of the less compassionate managers to end up giving me a task that'll keep me an hour past my leave time.
If you slack off in an obvious manner... such as staring at the computer screen or a rack of discount boxers for five minutes, someone will surely notice and grouse at you. However, if you "seconds slack" it's much less noticeable.
So hang up a shirt, count five seconds, and hang up another shirt.
It's not enough time at once to seem wasteful, but it adds up and uses up that little bit of time that is too little to start a new task, but too much to finish the one you're completing.