While you are packing, make a list of all the meat, fruit, vegetable, alcohol and tobacco products that will be crossing the border. More information is always better than less. If you have it written down and can hand it to the agent, they are less likely to assume you are hiding something and request to search your vehicle.
Have your passports ready for everyone in your vehicle. If you have kids in the back seat, roll down the back window as they will want to see them.
Talk to your children before driving up to the agent. Let them know that this is a serious matter and that turning off electronics and being quiet is essential. Also explain that agents often talk to the children in the vehicle. Remind them that they should answer politely but keep their answers to the questions asked.
Have a travel letter for any children who are not traveling with both of their parents.
Have vaccination records for any pets traveling with you.

Get the car, trailer or rv unpacked right away. The longer you wait the bigger of a job it will seem. This will also eliminate the potential for food to spoil and stop clothing and other items from smelling bad.

Research what food you can and cannot take in to the US before you pack. If you are unsure, give the border crossing you will be using a call. Each crossing follows the rules of the state it is in. Different states have different rules about what fruits and vegetables can be brought in to their borders.