DO AS MUCH AS YOU CAN AS EARLY AS YOU CAN.
In other words, don't put anything off until the last minute.
For example, a very important thing a lot of people forget is thank you cards to send out after the event. The day after the event, you're going to have a lot of thank you cards to write. That's why I like to prepare them in advance by making sure I have them, writing return addresses, making a list of who to send them to, and writing some basic verbiage that can be changed for each note. This will save me a lot of time (and stress) the day after when I want to get the thank you cards out within 24 hours of the event.
One note: thank you cards really mean a lot to people but isn't using paper not very environmentally friendly? Buy post-consumer recycled cards! If you don't have an address for someone and need to send a thank you by email, I suggest using http://www.care2.com/ecards/. They give a donation to save the rainforest for each card you send!