Come conservare la Green Card
Al momento in cui si diventa residenti negli USA, dovete cominciare a registrare le entrate e le uscite dal paese anche per periodi brevissimi tipo un w/e in Canada o Messico. Il motivo per farlo lo capirete tra 4 anni e 9 mesi: quando farete la domanda sulla naturalizzazione, vi chiederanno di indicare tutte le assenze.

Se volete assentarvi per periodi lunghi, tenete conto che questo rallentera' la vostra domanda di naturalizzazione e potrebbe anche compromettere il vostro status. Quello che segue e' un articolo interessante sul tema, uscito in un link sul Forum; leggetelo attentamente.

C'e' anche questa pagina della American Chamber of Commerce in Italy da leggere
http://www.amcham.it/Italian/imm_green_faq.asp (registrazione gratuita richiesta)

How to Keep Your Green Card After You Get It
Follow these rules and you'll never lose your status as a permanent resident of the United States. 

Once you receive a green card, there are a few conditions required for you to keep it for life. For one thing, you must not violate certain criminal or immigration laws -- including one law that requires you to advise the immigration authorities within ten days if you change addresses. For another, you must not abandon the United States as your permanent residence.

If You Violate the Law
The most common way that people lose their right to a green card is by committing a crime. Unlike what is commonly believed, it doesn’t have to be a major crime or a felony. For example, a person can be deported for helping someone enter the United States illegally, for committing domestic violence, for possessing even a small amount of drugs, or for any crime that’s considered morally wrong (such as fraud, theft, a crime with the intent of doing great bodily harm, or a sex offense). Some of these crimes are misdemeanors that may not be punishable with time in jail.
However, there is no set list that tells you which crimes make you deportable. The criminal grounds of deportability are too complex to analyze on your own. If you are arrested for anything at all, consult not only a criminal lawyer, but also an immigration lawyer to find out whether and how you can avoid deportation.
In addition, a person can be deported for certain violations that don’t fall under the criminal laws. For example, if U.S. Citizenship and Immigration Services (USCIS, formerly called the INS) discovers that you got your green card through a fraudulent (sham) marriage, or any other type of fraud, you can be deported.
An immigrant can even be deported for failing to advise USCIS of a change of address within ten days of moving. In the past, USCIS almost never did anything about this. However, with increased security concerns, USCIS has begun using this rule against people it wishes to remove from the United States. To advise USCIS of your change of address, use Form AR-11, available on the USCIS website (www.uscis.gov).

If You Abandon Your U.S. Residence

It is a common misconception that to keep your green card all you need to do is enter the United States at least once a year. The fact is that if you ever leave with the intention of making some other country your permanent home, you give up your U.S. residency when you go. The border officials will look at your behavior for signals that your real place of residence is not the United States.
As a general rule, if you have a green card and leave the United States for more than one year, you may experience difficulty reentering the country. That is because the U.S. government feels that an absence of longer than one year indicates a possible abandonment of U.S. residence. Even if you do return before one year is up, you may run into trouble. To avoid a full-scale inspection, you should return within six months.
On the other hand, remaining outside the United States for more than one year does not mean you automatically lose your green card. If your absence was intended from the start to be only temporary, you may still keep your permanent resident status. However, you may no longer use your green card as a U.S. entry document. You must have what is known as a reentry permit, or you must apply at a U.S. consulate for a special immigrant visa as a returning resident.
Reentry Permits
Reentry permits are for people who hold green cards and know in advance that they must be outside the United States for more than one year. Under such circumstances, USCIS can allow you to stay away for up to two years. You should send in your application before leaving. Use Form I-131, available on the USCIS website at www.uscis.gov. If approved, a reentry permit will be issued and will serve as an entry document when you are ready to return.
Reentry permits cannot be renewed and can be applied for only inside the United States. If you want to stay away for more than two years, you must return briefly and apply for another reentry permit.

Returning Resident Visas
If you stay outside the United States for more than one year and do not get a reentry permit before leaving, you must apply at a U.S. consulate abroad for a special immigrant visa as a returning resident. You must convince the consular officer that your absence from the United States has been temporary and you never planned to abandon your U.S. residence. You will have to show evidence that you were kept away longer than one year due to unforeseen circumstances. Such evidence might be a letter from a doctor showing that you or a family member had a medical problem. If you do not have a very good reason for failing to return within one year, there is a strong chance you will lose your green card.
The Commuter Exception
Green card holders who commute to work in the U.S. from Canada or Mexico on a daily or seasonal basis may keep their cards even while actually living outside the country. USCIS will grant you commuter status if you advise them of your intention to live on the other side of the U.S. border.

File for Citizenship to Avoid These Problems
You can lower the chances of losing your residence in the United States by applying for citizenship as soon as you are eligible. The waiting time for eligibility is usually five years after you get a green card, but there are exceptions: For example, the wait drops to four years if you received political asylum, and to three years if, at the time you got your green card, you were married to a U.S. citizen, and you're still married and living together.