Travel Insurance
Your regular health insurance may cover little or nothing abroad, and travel insurance is what plugs that gap.
Packing Light
Light packers lay out everything they think they need, then put about a third of it back before the suitcase ever opens.
Currency Exchange
That friendly offer to "just charge it in dollars" from a foreign merchant is almost always a worse deal than simply paying in the local currency.
Travel Vaccinations
Only yellow fever is legally required for entry in some countries; nearly every other travel vaccine is merely recommended.
Jet Lag
The same total number of hours flown can produce dramatically different jet lag depending on which direction the plane happens to be heading.
Passports & Visas
A valid passport can still get you turned away if it expires too soon, since many countries want about six months left.