Have you researched the places you intend to travel to and studied good-quality, up-to-date travel guides for those areas?
Have you checked that the country you're planning to visit is currently politically stable and not hostile towards western visitors?
Have you made arrangements so that family or friends can contact you in the event of an emergency? Do they know where you're staying - particularly if you're on a touring holiday? Do you plan on making occasional phone calls (or emails) home so that people know you're safe and don't worry about you?
Will you be able to use your mobile/cell phone in the area you are visiting and what will be the likely costs if you do?
Have you checked that you're on a payment plan that allows you to use your mobile/cell phone abroad?
Do you need to learn some of the local language in the area you're visiting? Will you be able to get by by just speaking English?
Have you remembered to get the appropriate phrase books?
Are you aware of any need to ensure that your dress and manner 'appropriately reflect your surroundings'?
Is it recommended that you get any particular vaccinations/innoculations for the countries you intend to visit?
Do you have adequate supplies of prescription medications?
Do you need to take a copy of your prescription?
Are your prescription medications legal in the countries you intend to travel to?
Have you had a dental check-up to ensure there are no obvious problems looming?
If your holiday involves a long-haul flight, do you have some deep vein thrombosis socks and a plan to keep moving on the aircraft (via walking round the cabin, regular stretching and following the exercises to be found in the airline magazine?)?
If you have specific food restrictions, do you have a plan to enable you to observe them whilst you're on holiday?
Have you assembled a basic first aid kit? - antacid tablets, anti-diarrhoea tablets, laxatives, painkillers, bandages, plasters, antiseptic wipes, etc
Have you organised adequate health and travel insurance? Do you know the limits of your health and travel insurance?
Are you aware of the standards of medical care you can expect to receive in the areas you intend to visit? (This may have a bearing on the amount of travel insurance you need to get.)
Have you remembered to declare ALL pre-existing illnesses on your travel insurance? Failure to do so can result in loss of cover.
If you're planning lots of trips, would it be cheaper to get a year-round insurance policy?
Have you made two copies of your passports, insurance details, travel insurance policy, ticket details and itinerary and left one of these with family members or trusted friends?
Is your passport still valid? Will it be valid for the duration of your trip? (And some countries require that your passport be valid for a certain length of time AFTER your trip.)
Will you need a visa or to complete visa waiver forms before you set off on holiday?
Do you have a separate list of emergency contact numbers for all your credit cards and bank cards?
Have you told your credit card provider that you're likely to be using your card abroad? (I'm told that they particularly appreciate you doing this and it can stop any temporary withdrawal of service (and the accompanying embarrassment of a rejected transaction) whilst they contact you to check out any unusual activity.)
If you plan to travel abroad for extended periods of time, do you have a plan for accessing your bank accounts and paying your credit card balances from wherever you are in the world?
If you plan on travelling with a pet, do you have proof of immunization in case you're asked for it?
Have a good trip!