What to pack?

The age old question, we tend to pack light, yet still end up with things we don't use. So for a week or more trip (this is the same list we'll use for 6 weeks) here's what to pack (for 1):

• 2 x pants
• 1 x shorts
• 1 x dress
• 4 x shirts (usually long sleeve cotton)
• 7 x socks
• 7 x undies
• 1 x pj
• 1 x sandal
• 1 x walking shoe
• 1 x hiking boots (if hiking)
• 1 x swimsuit
• 1 x sports towel
• ipod
• 2-3 books
• Medical kit
• Travel docs
• Travel diary
• 2 x pens

What do you make sure you always travel with? And what do you leave at home?