There are multiple ways of doing this ie using JavaScript, but this is the server side solution.

You can adapt this method for several other solutions:

        foreach (Control c in this.Page.Controls)
        {                        
            if(c.GetType().FullName.Equals("System.Web.UI.HtmlControls.HtmlForm")) {
                foreach (Control cForm in c.Controls)
                {                        
                    if(cForm.GetType().Equals(typeof(TextBox))) {   
                        ((TextBox)(cForm)).Text = "";
                    }
                }
            }
        }

If you would like your application to set the selected item in your dropdownlist to an item from a variable passed to it, or from the database

Here are some examples:

ASP.NET 

<asp:DropDownList ID="DropDownList1" runat="server"/><br />

DropDownList1.SelectedIndex = DropDownList1.Items.IndexOf(DropDownList1.Items.FindByValue(value.ToString()));

VBScript ASP Example:

  response.write "<select size=""1"" name=""selectData"" id=""selectData"">"
  response.write "<option value='0'>-- Select  Data --</option>"
  do while not objRs.eof

      if (ContactType = objRS("Value")) then
          response.write "<option value='"+cstr(objRS("Value"))+"' selected>"+objRS("Description")+"</option>"
      else
          response.write "<option value='"+cstr(objRS("Value"))+"'>"+objRS("Description")+"</option>"
      end if

      objRs.MoveNext
  loop
  response.write "</select></font></p>"

If you have a lot of variables i suggest using a for loop:

    response.write "<select name='select1'>"

    Dim MyWebSiteArray
    PrefOddsArray = MyWebSiteArray("web design","design","website","nathan","baker")

    For each Item in MyWebSiteArray

   DatabaseValue = RS("ValueFromDatabase")

   if(DatabaseValue = Item)  then
        response.write "<option value="""+Item+""" selected>"+Item+"</option>\n"
    else
        response.write "<option value="""+Item+""">"+Item+"</option>\n"
    end if
    Next
    response.write "</select></td>"
    response.write "</tr>"

Configuration Error
Description: An error occurred during the processing of a configuration file required to service this request. Please review the specific error details below and modify your configuration file appropriately.

Parser Error Message: The entry 'ScriptModule' has already been added.

Source Error:

<add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>


This happens when the config for a virtual directory is already defined in the root web application

There are two ways you can fix this:

<remove name="ScriptModule" />

Alternatively you can stop the  the web.config inheritance to the the virtual directory.
by adding this to the parent web.config

    <location path="." inheritInChildApplications="false">
    <system.web>
    ...
    </system.web>
    </location>

I found out that the root application had .NET 3.5 references in the web.config

I tried to remove this using:

    <location path="." inheritInChildApplications="false">
    <system.web>
    ...
    </system.web>
    </location>


the problem actually occured due to the  3.5 reference in the <configSections>

Configuration Error

Description: An error occurred during the processing of a configuration file required to service this request. Please review the specific error details below and modify your configuration file appropriately.

Parser Error Message: The entry 'ScriptModule' has already been added.

Source Error:

<add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>


This happens when the config for a virtual directory is already defined in the root web application


There are two ways you can fix this:

<remove name="ScriptModule" />

Alternatively you can stop the  the web.config inheritance to the the virtual directory.
by adding this to the parent web.config

    <location path="." inheritInChildApplications="false">
    <system.web>
    ...
    </system.web>
    </location>


--------------------------

I also came upon this problem when trying to fix this issue:

Unable to cast object of type 'System.Web.Configuration.ScriptingAuthenticationServiceSection' to type 'System.Web.Configuration.ScriptingAuthenticationServiceSection'.

which led me to realise there were .NET 3.5 references in the parent application's web.config

I tried to remove this using:

    <location path="." inheritInChildApplications="false">
    <system.web>
    ...
    </system.web>
    </location>

the problem actually occured due to the 3.5 reference in the <configSections>

Unable to serialize the session state. In 'StateServer' and 'SQLServer' mode, ASP.NET will serialize the session state objects, and as a result non-serializable objects or MarshalByRef objects are not permitted. The same restriction applies if similar serialization is done by the custom session state store in 'Custom' mode.

When receiving this error you will need to make your object Serializable.

 ie:

[Serializable]

public class MyClass
{

}

Alternatively you can use the InProc Session State.