To prevent the occurrence of pigtail marks or swirls during sanding with an orbital sander, it is important to consider several factors. 

First, it is crucial to control the suction speed of the extractor and the sander, ensuring that the speed is medium and never at full capacity. Additionally, pressure should not be applied to the sander. The choice of sandpaper grit is also significant, with a coarse grit like 80 or 100 being preferred for removing defects on wood. It is important to check that no grains are detaching from the paper before proceeding to a coarser grit like 60. It is also recommended to avoid skipping sanding grits on hard or very hard woods. 

Lastly, cleaning the sanding pad perfectly when changing the paper can help avoid sanding grains. By following these best practices, one can avoid encountering pigtail marks or small spirals and achieve good results in sanding with an orbital sander.