I assume you mean not to contaminate your DNA. Some suggestions are: use gloves and clean tips, spray bench, gloves and pipet with 70% ethanol, use your own reagents, stay on clean surfaces, store DNA at -80C (except during ethanol precipitation at -20C), if extracting with phenol, check that phenol is clear and not too old, autoclave your reagents, add RNase if your agarose gels show unexpected low molecular weight material, do not reuse electrophoresis buffers.  Best wishes.

The best practice is use separate hood for sample preparation, DNA extraction and PCR cocktail preparations seperately.

Use sterile Milliq and One specific clinical samples at a time.

It would be better to answer if you specify the sample or specimen from which you going to extract DNA,

Because protocols vary between different specimens.

Also above Albino points were go to consider basically.

thanks alot ,for your answer

Why if precipitation by ethanol must be store at -20 not -80