Every country has the right to protect their borders and it's crucial that you adhere to the rules and regulations of the countries that you plan to visit and your own. If you don't find out about what is required before you go, then it can bring about an abrupt stop to your journey and you may even risk imprisonment or a hefty fine.
My tips for Customs and Visas is this - make sure that you are applying for a visa or seeking information about customs on an official government site for the country you are visiting. The first few results in a search on the internet could be immigration advisors or lawyers, which you don't need unless you have some particular circumstances. You could end up paying triple the cost for a visa that is simply available at the government site or when you arrive in the country.
Things change and you can't assume that everything remains the same as when your friends travelled, even if it was last week. Keep yourself informed. An Australian or a New Zealander should check the amount of time you can spend in Europe. It used to be 90 days in each country - long ago, but now it is 90 days in the entire Schengen area for Aussies and 180 days for Kiwis, (but check it out, it may have changed again). When you travel to the USA, you need to have an outward going ticket to a country other than either Canada or Mexico. They won't let on the plane at the airport if you don't have one. They also tell you that you don't need to print off your ESTA but I've had to show it in foreign countries before they've let me on the plane to go to USA. I was happy to have my copy on hand. Once again, check in case it's changed.
Customs - island nations are special and are extremely strict about goods being brought into their country. You will risk heavy fines if you ignore the warnings and try and bring in fresh produce. Most commercially made products are able to be brought in, but you must declare them as there are exclusions, such as honey and seeds.
Going through customs in Peru was a bit different. They have a random red/green light system. If the light turns red for you then you may be thoroughly searched. If the light is green - you walk right through.
Some countries have the death penalty for trafficking drugs or you could end up in a jail at the end of the earth. Just don't ever risk it!!!! And always know what is in your bags.
Some prescription drugs are illegal in some countries. Make sure you are able to take any of your prescribed drugs along with you and make sure you have a letter from a doctor.