How do I clear the server cache in asp.net?

You could loop through all the cache items and delete them one by one:

foreach (System.Collections.DictionaryEntry entry in HttpContext.Current.Cache){
    HttpContext.Current.Cache.Remove(string(entry.Key));
}

Syntax Correction for ASP.NET 4.5 C#

foreach (System.Collections.DictionaryEntry entry in HttpContext.Current.Cache){
    HttpContext.Current.Cache.Remove((string)entry.Key);
}