Drspaniel is mostly right... but it is not just about posting high quality content... it is important that you know your subject well. There is a difference between writing a really great hub and writing a book report. Take the subject farther by making it your own. Write about the topic from your perspective. Be intimate about what you write. Use your own pictures and videos... this really makes a big difference in both how Google sees your hub and how hubpages sees your hub. Use the hubpage tools available to you to make your hub look great. Think about high quality magazine articles... that is the look you are shooting for.
There is a process for how hubs are chosen for hub of the day. The basic entry requirement is that you publish your hub. There are a lot of hubs that could be hub of the day but are not chosen because the photo credits or video credits are not correct. Read through your hub several times and don't be afraid to let it sit for a few days before you publish it.