When you are writing things out, you can actually use format converters to control what is displayed. It's normally a bad idea though to force a date to appear in a format other than the one the user is expecting - this often leads to confusion. Suppose I put 11/05/2010 in as a date, is this the 11th of May or the 5th of November? Suppose that my appointment is for the 11th of May, but I miss it because I've read the date wrong. Simple things like that can really ruin the user experience and are best avoided.
If you must force the change though, you can use the format specifier
{0:dd/MM/yyyy}
, e.g.
string date = string.Format("{0:dd/MM/yyyy", DateTime.Now);