The most important step in keeping a man faithful is the first step: Find a man capable of being faithful. Not all men have this ability. To determine if your guy has it, you will need to know how to do some serious investigation.
Start by asking about his family. Are his parents still together? What kind of relationship do they have? What about the siblings? Any divorces? Any infidelity? If so, get him to tell you about them and what he thinks about such behavior. This may take several weeks, but the information you garner will be well worth it. Of course, you may pick up strong evidence early on that this fellow does not meet your criteria. If he tells you his favorite uncle's wife divorced him after one "drunken indiscretion" and then goes on to say something like "most men are not made to have just one woman," then you can just stop and move on to the next candidate.
If, however, he talks about his belief that people should live according to a code of honesty, openness and fidelity, you may have a keeper. But keep investigating because behavior says more about a person's values than words say. And keep in mind that even if his parents are divorced, it is important for you to understand what his thoughts are about this and how it has impacted his own thoughts about marriage and fidelity. At this point you probably don't have to inquire about his feelings. If he is like most men, he may have thoughts and feelings confused and is more comfortable talking about thoughts anyway. Of course, once you meet his family, you can continue the investigation with them. The women will be able to fill you in on what kind of guy he is, the extent of his emotional intelligence, and any quirks you might need to explore further. You don't need to ask a lot of direct questions, but rather listen to their family stories and be especially alert for the tone of voice, rolling eyes, and other body language give-aways.
After about three to six months if you are still comfortable that this may be the one for you, you can start to settle into a committed relationship. You will need to talk to him about this and be sure you are on the same page regarding the commitment. You need to know how open he is to talking through disagreements and how important it is to him that he understands your point of view. Another important consideration is how willing he is to being accountable. Whether it is about finances, commitments to relatives or sharing household duties, find out just how he reacts to being accountable to you.
Men stray for a number of reasons. Sometimes they think they are just made that way and it is their nature and their right. Sometimes they simply slip up and fall into an affair before they know what hit them. If you hook up with the first type, you will not be able to keep him faithful. If the second, you and he will need to keep communication open as a way of protecting your relationship because at some point the honeymoon will end and the work of making a great marriage begins.