Besides making sure your passport is valid for at least six months before travelling internationally, make sure it's not too full. Some countries require you have several blank pages before you leave home.
Image from Junpei Abe
Some countries require between two and four blank pages in your passport for visas before you enter the country. South Africa, for example, requires two blank pages. You can check if the country you're visiting requires blank pages -- and if so, how many -- on the Smart Traveller website -- choose your country and look at the entry and exit requirements.
Passports & Travel Visas [Friendly Planet Travel]