best way to persist data in .NET Web Service

Context.Cache.Insert("foo", _
                 Foo, _
                 Nothing, _
                 DateAdd(DateInterval.Minute, 30, Now()), _
                 System.Web.Caching.Cache.NoSlidingExpiration)

<WebMethod(CacheDuration:=60)> _
Public Function HelloWorld() As String
    Return "Hello World"
End Function

public class WebServiceClass
{
    private static DataTable _myData = null;
    public static DataTable MyData
    {
        get
        {
            if (_myData == null)
            {
                _myData = ParseJsonDataReturnDT();
            }
            return _myData;
        }
    }

    [WebMethod]
    public string GetData()
    {
        //... do some stuff with MyData and return a string ...
        return MyData.Rows[0]["MyColumn"].ToString();
    }
}