First of all, it depends on the source of your genomic DNA!! 

please clarify by some examples............

Hello, do you use tissue, soil, or fecal samples for example? Can you specify the sample type you are using?

If your samples are tissue then phenol/chloroform extraction method is very efficient. let me know if you are using different samples.

DNA extraction involved 3 steps:1- cell lysis by destruction of cell wall and other membrane. 2- precipitation of protein. 3- recover of DNA and re-precipitate it by isopropanol. you can depend on molecular cloning a laboratory methods 2001 to Sambrook and Russeel for DNA extraction. with my regards  

Follow the link for a very general DNA extraction protocol. 

Tissue homogenization/cell lysis approach would need to be adjusted depending on your sample of interest. 

http://www.funpecrp.com.br/gmr/year2011/vol10-1/pdf/gmr1055.pdf

There are multiple methods.For you I  will refer Ethanol precipitation,because its easy and have the minimum cost, besides this, there are also many commercial kits to purify DNA ..I AM SURPRISED YOU ASKING ABOUT PURIFICATION AND ALL THE ABOVE COMMENTS ARE ABOUT ISOLATION, STRANGE..

you can use  Agencourt™ AMPure™ XP beads

There are many methods indeed to purify DNA but surely depend on the sample source you extract it from. If you are interested in getting pure DNA then you might want to add an RNase treatment to your extraction protocol. Of course the treatment can come together with some extraction kits. Also Ethanol precipitation works best with genomic DNA and reduces the chance of salt contamination in your samples compared with isopropanol precipitation.

The fundamental processes of DNA purification from cells and

tissues are sample lysis and the segregation of the nucleic acid

away from contaminants.While DNA is more or less universal to

all species, the contaminants and their relative amounts will differ considerably.The composition of fat cells differs significantly from

muscle cells. Plants have to sustain high pressure, contain chloroplasts

packed with chromophores, and often have a very rigid

outer cell wall. Bacteria contain lipopolysaccharides that can

interfere with purification and cause toxicity problems when

present in downstream applications. Fibrous tissues such as heart

and skeletal muscle are tough to homogenize. These variations

have to be taken into consideration when developing or selecting

a lysis method.

DNA extraction involved 3 steps