Avoid visiting too early or too late
The first and last months of a World Expo are far from ideal for a visit, unless you're planning to attend the opening or closing ceremonies.
During the first month, the organizer and participants learn how things operate. Several pavilions and attractions will be unfinished, stores will be missing many products, and signs will be missing or hard to understand. However, if you don't mind things not fully operating, you'll have the advantage of exploring the Expo site with very few people.
During the last month, people rush to visit the Expo, and almost everything gets crowded. Lines are long, the hosting staff is tired of thousands of visitors asking the same questions, and many things that stop working probably won't be repaired.
Don't plan to visit a pavilion during its country's National Day
Official participants in World Expos are assigned a National Day. Pavilions usually receive high-level delegations from their country during their National Day, and access to their exhibitions tends to be open only to special guests.
Long lines do not guarantee good exhibitions
Do some research about pavilions before spending too much time in lines. Sometimes, the buzz around a pavilion isn't justified, and you'll feel disappointed for having missed the opportunity to explore other more interesting options, especially if you're only visiting for a couple days.
Interact with hosting staff
The most undervalued asset of national pavilions is hosts brought from their countries to attend visitors. No interactive exhibit will provide you a better experience than engaging in an dialogue with a human who can share different perspectives of the world.
Some countries try to save money by hiring local staff to attend visitors, but this frequently leads to missed opportunities to communicate the richness and complexity of a country.
Please remember to be nice to hosting staff. They attend thousands of people every day, some of which try to cheat their way into the pavilion's front of the line, or aren't careful with the exhibits. It can be an exhausting job.