Make sure that your attorney has accurate, up-to-date basic information on both you and your fiance. this includes your telephone numbers, email addresses, and mailing addresses. If you don't keep you information up-to-date you may miss important communication from USCIS and your attorney.
Disclose your immigration history
Your attorney needs to know if you (the US citizen or LPR) have ever filed any petitions with USCIS in the past. make sure your attorney knows what your filed, when you filed it, and who it was for. For instance, If you never disclose to the attorney that you previously filed two k-1 visa petitions then your attorney cannot properly tell you that you will need a waiver or to file an I-130 instead.
It you are a naturalized citizen, your attorney needs to know how you got your immigration status (through marriage, work, family, etc.). These answers could affect your ability to file for certain immigration benefits.
Disclose your criminal history
Certain types of visas require waivers if the (US citizen or LPR) has a criminal history. This includes some felonies and misdemeanors. You need to disclose this information to ensure that it will not be a ground for denial of your petition.
Disclose your income
Your attorney needs to know how much money you make, how many dependants you have, and how many people you intend to petition for (in other words, is your fiance bringing children, will your brother bring his family of 5?) These issues directly affect your ability to sponsor immigrants.
Disclose the intending immigrant's criminal history
You have to tell your attorney if the beneficiary has a criminal record so that the proper paperwork can be prepared.
Disclose the intending immigrant's immigration history
You need to tell your attorney if, when, and how the beneficiary has ever entered the US. You also need to tell the attorney if the beneficiary has ever been denied a visa, asylum, or other US immigration benefit. Failure to disclose these items could result in a denial because your attorney did not prepare the right waivers.
Disclose the intending immigrant's health issues
If your beneficiary has serious illness you need to let your attorney know as a waiver may be available.