You likely know already that applying for a visa to stay in the U.S. is not going to be easy. However, people do get these visas, so don't let the pile of paperwork discourage you. If you take the application one section at a time and don't rush, you should be able to complete the application correctly. That being said, there are some errors you want to avoid at all costs.
Any Hint of Fraud
Immigration officers who review visa applications are on the lookout for potential fraud, especially in people claiming to be coming over for marriage to an American. If you really are coming over here to marry an American, you and your American spouse-to-be need to come up with very good proof that the relationship is legitimate. For example, you can use emails, letters, photos of you two together from the past few years, and other forms of proof that show the marriage is for real and not just to get you a green card. Be prepared for tough questioning. This is not because the officers want to single you out, but because there is a problem with marriage fraud, and they need to be careful about who gets a visa.
Any Hint of Work (for Non-work Visas) or Reasons to Overstay
If you're coming to the U.S. for a job or conference, chances are you're going to be applying for a work visa or another type of visa related to that. But if you're coming over here for tourism, school, or other fairly temporary reasons, you do not want to give the visa officers reviewing your application any reason to suspect that you might be trying to sneak in to earn money or that you might be a risk for overstaying your visa and remaining here illegally. Be honest about your reasons for wanting the visa, and be very careful about the language you use.
The Usual Mistakes
Don't get so wrapped up in describing your visit and gathering proof that you forget to simply fill out the form correctly. When you fill out the application, put it aside for a while and read it over the next day. Look for incomplete answers and missing documentation. Of all the problems that could derail your visa application, basic mistakes and missing forms are among the top.
It helps to contact an immigration attorney to discuss exactly how to fill out your application. You can reach many online, so you can try to get help no matter where you are.