In a word: EVIDENCE.
People get married for all kinds of reasons. Generally, under the immigration law, a marriage that is legally valid is not necessarily valid for immigration purposes. There must be an intent for the husband and wife to establish a life together. Of course, not every marriage works out, and there is nothing illegal about separating from your spouse, or divorcing him/her. But because of this extra requirement of showing that there was an intent to establish a life together, a foreign national may have trouble proving that the marriage was bona fide (true or real, as opposed to a "fake" or "sham" marriage).
Few people can admit that they have weaknesses in their case. No one wants to think that they may have a marriage that the government will view as suspect. I cannot count the number of times a client has said, "I have done nothing wrong, and I have nothing to hide." While I usually agree - that is simply not the question. The question is, "Can I translate my marriage story into hard, cold documents to prove to a suspicious immigration officer that my marriage was a real marriage?" A lawyer's job is not so much to fill out the forms, but to make sure his particularclient's story is told to the officer.
That requires evidence. How to obtain it, create it, find it, evaluate it, collect it, and present it - that is where your lawyer's skill becomes apparent. Your lawyer must take the time to amass the necessary evidence. Evidence you don't even know you have - or evidence you don't know you could get.
People overlook the fact that immigration is a lengthy, multi-step process. Just because you got your two year green card does not mean you will get your 10 year green card. Just because you got your 10 year green card does not mean you will get your citizenship in 3 years. Just because you're filing for your naturalization after having your green card for 5 years does not mean USCIS will not look at your marriage. Some people make it all the way, but there are a lot of people who run afoul of a suspicious immigration officer along the way and it causes untold misery for the husband and wife.
When the marriage is shaky - when there have been periods of separation, for example, or the husband and wife did not live together all the time, where there is vengefulness or bad blood between husband and wife, or things otherwise simply didn't go as planned - then special attention must be paid in constructing the case. There is no set way - each case is built differently because each story is different. The time to involve a lawyer is sooner rather than later. Most cases will fail on lack of evidence. If you don't have evidence, then the only evidence will be the evidence dug up by the government when they investigate your case. You cannot expect the government to see things as you do. Your immigration benefit is your burden of proof. If you fail, you don't get it. And worse, if USCIS thinks your marriage was a sham, they will make the allegation and it will then be your battle to defend against those charges, or face the permanent lifetime bar of INA §204(c) - which says that no other immigrant petition can ever be approved for you. This means you will never be able to apply for permanent residence, and you can even lose the green card you already have.
Lastly - if you did enter into a sham marriage, do not think you will be the lucky one to just slide by. You will probably get caught, because the Department of Homeland Security has a lot more experience than you do. The system is set up so you have to check in with them every so often, so they have plenty of opportunities to sniff you out. If they smell fraud, then perhaps the following quote from an immigration officer can be of service: "I love catching people in a lie."