Your destination will ultimately determine what items are essential and what are frivolous for packing. Just bear in mind that overpacking is a wasteful pursuit. These days airlines upped their charges on over-weight luggage and extra bags fees so I suggest you packed to a minimum. Well, it might be the most difficult chore, but it is necessary. Packing your clothes is the most tedious task. My suggestion is you double up your outfits based on the number of days you're staying. Buy souvenir T-shirts or other apparels and use them! You will be buying souvenirs anyway so take this as an option to lighten up your load before heading to your destination. The extra space in your luggage should cater for all the souvenirs you're going to buy from the place that you're going to have your vacation.
As for women, don't pack your entire beauty regime into your suitcase. Go with the flow on vacation. Grab a few travel-sized items from your local store and rely on those to get through your vacation. Most hotels supply toiletries like shampoo and conditioner, shower gel, toothpaste, etc., so use them! They are complimentary and you can always get more if you run out. They are not for you to keep as souvenirs.
Don't forget the most essential gadget - your camera
The above is not my comprehensive list. They are just some of the common things that I suggest you probably need to pay attention when packing your suitcase. I would be very glad if you can share with me your own packing tips so please feel free to write a comment on this post for your suggestions and opinions.