Discussion on:

Opening form instances in C#

How solve the flickering problem

Keyboard form connect to text form

YOU MUST STOP THE REPEATE (IF WE USE THIS BUTTON MORE THAN ONE TIME)

It may be old code but still works well. I did modify it slightly to make the Instanceref of the type of the form. This gives it reference to the forms methods without needing to make it static. Of course the forms needs to be instantiated at run time before Form2 is called but in my code Form1 is called via Application.Run(Form1) from Main() so is always available.

e.g.
private Form1 m_InstanceRef = null;
public Form1 InstanceRef
{
get
{
return m_InstanceRef;
}
set
{
m_InstanceRef = value;
}
}

how about
Form1 frm= (Form1)Application.OpenForms["Form1"];