Hello!

First, when you design primers, try to make them annealing only your target organism (especially be aware of possible annealing on human templates).

And, of course, use gloves and filter tips, separate sample preparation and amplification rooms.

use a separate pcr area from the gel running area and filter tips as Iryna says.

Always run a no template control so you will detect contamination early. Be very careful opening plates/tubes after pcr as opening causes aerosol droplets to spread and this is one cause of contamination. Another cause is electrophoresis. .Picking the gel up wets the gloves with buffer which always has some pcr product from the gel in it then the buffer dries on the gloves and clothing and anything handled and spreads in the dust in the lab working area. Use different pipettes for loading sample from those used in pcr preparation. Always remember that 20 pcr cycles is 1,000,000 times more product so even microscopic droplets or contamination will completely take over a pcr reaction

you should work on a seperate area of nucleic acid extraction , PCR loading and electrophoresis.