Here is a list of the many things you should check, pack and do before you go away on holiday.  We cannot guarantee that it is exhaustive, but it should help you to remember the essentials.

Make arrangements for pets
Cancel papers, milk, cleaners, medical appointments, home visitors etc.
Confirm flights, ferries etc
Check passports and travel insurance are valid
Arrange visas where necessary
Check if any inoculations are needed for the countries where you are travelling
Arrange taxis if necessary
Check tickets
Obtain foreign currency
Check that you have enough medicines for your time away + a little extra
Notify the police and neighbourhood watch of the dates that you are away + contact details
Pack!
Advise your bank and card companies where and when you are going away, otherwise they may block your cards for 'unexpected transactions'.

If taking your car abroad, have it serviced, notify your insurers, get AA cover or something similar and check you have the required items for the countries where you are driving eg breathalysers in France, warning triangles, fluorescent vests etc.

Things to take

Passport
Travel Insurance and contact details
Tickets
First aid kit including paracetamol, birth control, plasters, insect repellant, sanitary products, ointment, travel sickness tablets
Driving license (both parts)
Visas - where necessary
Medicines
Eu insurance card
Sun cream and lip balm
Toilet  bag
Make-up bag
Suitcases (it has been known!)
Sunglasses, contact lenses and/or spare specs
Mobile phone (check with your phone company on usage abroad)
Laptop and/or tablet
Cameras
Chargers and adapters
Clothes and Shoes
Hat(s)
Your wallet/handbag

If you are travelling as a couple/family, share your clothes between cases so that if a case goes missing you both still have something to wear
Take a couple of spare plastic bags for any wet or soiled clothing

If travelling with children

Activity books/toys
Favourite teddy
Travel sweets
Sense of humour!

With a baby

Nappies and changing mat
Nappy cream
Baby wipes
Teething ointment
Bottles of milk - check quantities with airline
Change of clothes
Blanket
Dummy/comforter
Baby sun hat
Baby sun cream
Baby food
Passport

Last minute things to do

Check that you have all of the above
Lock the door
Leave a key and contact details with a neighbour or friend
Turn off heating (or leave on low in winter)
Unplug all electrical appliances
You may choose to put a couple of lights on a timer
In winter turn off water at main tap

Lastly, leave all your troubles behind, relax and enjoy your holiday!