For example

Your current code in the method of the thread looks like

public void ThreadMethod()
{
....
SubForm form = new SubForm();
...
form.Show();
...
}

Rewrite it to

public void ThreadMethod()
{
...
CallFormShow(form)
...
}

protected delegate void CallFormShowDelegate();
protected void CallFormShowInvoke()

protected void CallFormShow()
{
mainForm.Invoke(new CallFormShowDelegate(CallFormShowInvoke), new object[]{});
}

protected void CallFormShowInvoke()
{
SubForm form = new SubForm();
...
form.Show();
}

Hope this could help you.