When putting the cap aside place it on the rim if possible. This ensures that nothing falls into the cap due to careless movements and prevents the risk of contamination from particles on the basement plate. Place plate and dish lids either inside-down or inside-up at a position with minimal movement to minimize the risk of contamination.
Putting a lid at a higher position such as on a pipette tips box, however, provides no additional protection against contamination. Only open sterile vessels immediatly prior to use and reclose them immediatly after use. There is a certain air flow within the hood that will keep the work area as germ-free as possible.
Do not fill the interior with too many items to ensure uniform air flow. Furthermore, avoid unnecessarily quick movements and air turbulence, e.g. caused by open flames (Bunsen burner). Disturbances to the air bring particles and germs into unnecessary movements.
Learn how to fight cell culture contamination on www.eppendorf.com/cellexperts