One simple way is using on GotFocus and LostFocus. On GotFocus the code should be something like this:
if (textbox1.Text == "My default text...")
textbox1.Text = "";
Just to make sure to no delete a text previously entered by the user. And on LostFocus just something like this:
if (String.IsNullOrWhiteSpace(textbox1.Text))
textbox1.Text = "My default text...";
So again you are checking to no delete a user entered text.
Hope this helps