Men wear lederhosen and women wear dirndls.
Wearing normal clothes to Oktoberfest was my mistake. I didn't splurge on an authentic dirndl (about $100-200). Almost everyone (including all the Germans) will dress up in authentic costume. You will never find a German not in lederhosen or a dirndl.
It's fun to get into the spirit of the festival, and one way to do so is the traditional dress!
2. Get to the tents in the morning (and get there early)
There will be huge crowds outside the tent entrances before they open at 9 or 10 am.
Be prepared to get your foot stepped on, elbowed in the side, or pushed around a little.
Don't worry - once you get in and find a table you won't have to deal with this crowd again! (That is, unless you leave the tent and try to re-enter...and try waiting for people to leave the tents at maximum capacity to be let back in).
3. Sprint to find a table
Once the doors open, everyone literally sprints to get a table. Tables fill up very quickly so just take whatever you can get.
Have your friends' phone numbers on hand in case you get separated so you can find them - the tents are quite large with thousands of people inside.
While a table reservation isn't required, you may do so beforehand by contacting the brewery.
4. Tip the ladies that serve you beer and food
They deserve it! Tip at least a euro per stein. They carry 10-12 steins full of beer at a time, all day long, for days on end. Also, if you don't tip them, they won't come back. And you definitely want them to come back, because it can get tough to find a new server during the chaos.
5. Pace yourself