While this technique can be achieved in post production with numerous shots, you can save time doing it in-camera while you are on location.
What you need:
1) Pentax Camera that supports Average Composite of Multiple Exposures (K3)
2) Tripod
3) Remote Shutter Release (not absolutely needed)
Technique:
1) Frame you scene on your tripod.
2) Place your camera in "Interval Composite" mode
3) Set Composite Mode to "Average",
- Uncheck "Save Process" if you do not want every photo in the interval
- Chose an interval between each photo (I recommend making the interval longer then the exposure length. So an exposure or 3 seconds at say f22 would prompt me to use an interval of 4 or 5)
- Chose number of shots (Choppy water or a crowded areas will require more shots to smooth water or ghost people)
- Chose the time to start the interval (Either select to start when you press the shutter button/remote or set a specific time if you don't have a remote and don't want to shake the tripod)
4) If you are photographing a scene with something like foliage that you want sharp, simply change your exposure mode and take a single frame that freezes that portion of the screen and composite your interval photo with it in post.