The hat you choose should depend on the look you are going for. For example, if the dress code (like ours said on Friday) is 'sporty', then face caps are your friend.
* Know which hat suits what look. On sporty day, i first wore my blue bowler-style hat but it made it dressy and clashed!
* Do not neglect your hair. Sometimes we slap the hat on and don't style our hair then you have to take the hat off for some reason and you look weird. Pack, pin, do something!
* Simplify. Hats can be dramatic so if you go heavy on your accessories and make up, it will have an adverse effect. Pair with simple accessories and lipstick and one dramatic item!