A few things:
1. You should not use Form.Show()/ShowDialog() to open/show a form in AutoCAD. Instead, use Application.ShowModalDialog()/ShowModelessDialog();
2. If the form is shown as modal dialog, Form.Close() DOES not dispose the form, it just sets the dialog form in invisible. When you call Form.Show()/ShowDialog() again, the hidden form simply becomes visible again, and the data on the form remain unchanged, unless you have code to update the form's data (you should!).
3. It is also important the command method that instantiates the form is static or not;
4. if your code involving a modal dialog form is gigantic, something is quite wrong (in term of code structure), unless you have a monstrous form with hundreds of controls on it (in this case, it may well also be wrong). You need to separate AutoCAD operation code from the Form's pure UI code.