Limpar campos dos textboxes.

Um pequeno exemplo de limpeza de campos que usam TextBoxes:

private void button1_Click(object sender, EventArgs e)
{
foreach (Control c in Controls)
{
if (c is TextBox)
{
(c as TextBox).Text = "";
}

if (c is Panel)
{
for (int i = 0; i < c.Controls.Count; i++)
{
if (c.Controls[i] is TextBox)
{
(c.Controls[i] as TextBox).Text = "";
}
}
}

if (c is GroupBox)
{
for (int i = 0; i < c.Controls.Count; i++)
{
if (c.Controls[i] is TextBox)
{
(c.Controls[i] as TextBox).Text = "";
}
}
}
}
}