What is the procedure to obtain greek citizenship for a non EU national married to a greek?

Depends.

The first option is naturalization if the non-EU national has no EU ancestry/descent/origin.
- Have a clean criminal record
- Be over 18 years of age
- Be married a minimum of 3 years
- Speak Greek fluently
- Live in Greece for 3-7 years
It takes years for these applications to be examined and processed. You can read all about it here, in which the process is detailed from beginning to end: "Greek citizenship by naturalization"
http://livingingreece.gr/2007/03/16/greek-citizenship-citizen-through-naturalization-american-spouse-greece-marriage-work-residence-permit/

The second option is to claim citizenship through an ancestor born in an EU country, if applicable.
- Often no language requirement
- Often no fee
- Often no residency requirements, in fact one can apply outside the country at a consulate or embassy
- Doesn't matter who you are married to

You can read all about it here:
"How to get EU citizenship by ancestry or naturalization"
http://livingingreece.gr/2008/03/18/how-to-acquire-eu-citizenship-through-ancestry-or-naturalization/