Having an address is a MUST! Other than that, file your taxes and make sure you come in every 6 months.
If you are gone MORE than 6 months they can review your "status". It could possibly end up in an immigration court. The biggest thing they will look at if they review your status, is what your INTENT is. Do you intend to return to the USA to live permanently? Do you have strong ties to the USA? Bank accounts, loans, house, family, car, investments, credit cards, memberships, medical insurance, etc.
If you are questioned by the Border Agents on arrival, you will need to convince them of these ties. If they suspect that you really have no ties or that the intent to return permanently is missing, they would most likely ask you to voluntarily surrender your green card and your status in order to gain entry into the USA. You would then be allowed to enter the USA via the Visa Waiver program if you are from an eligible country. If you are not eligible through the visa waiver program, you could actually be out of luck and refused entry.
If you refuse to voluntarily surrender your status, you could then be arraigned in front of the Immigration Court for a hearing, this might take weeks. During that time I do not believe you would ever be taken into "custody". I am not sure, but leaving the country under such circumstances might mean a big problem getting back in.
If you are worried about being away more than 6 months, then you should just apply for the re-entry permit. It is good for a year and can be extended for one more.
Every border entry point may act differently and every agent may concentrate on different areas. So there are no guarantees with anything.
It really would be smart to talk to an immigration lawyer on the subject. I talked to 2 in different jurisdictions and did a lot of reading on the laws and the interpretations. I am still not absolutely clear on the subject, a lot of it just comes down to your intent.