I went to Oktoberfest the previous 2 years. Arrive on a Friday morning so we could actually spend a little time in Munich as well as visit the Hofbrauhaus.
Saturday it's get in early and try to find a seat. Both years the weather was fantastic so didn't bother trying to get inside any tents, just sat out in the sunshine and waited for the mayor to crack the first keg. Manage to drink around 8 steins each day for the 3 or 4 days we were at Oktoberfest - and generally not have a hangover. Salivating at the thought of those delicious pork knuckles. The germans sure know which foods go well with beer, roast chicken, pork knuckle, salted fish cooked over coals, massive pickles, raddishes and huge pretzels. I wish I could have gone the hatrick.
I like the opening weekend, because everyone turns out for the saturday and sunday, including lots of local families. The monday it dies down as it's only tourists which kevo rightly points out are aussies/kiwis/saffas. All the locals I spoke to said they don't come on the 2nd weekend due to the Italians being extremely rude especially to women. Not really sure which is the worse of the 2? It was quite emarassing seeing all these aussies just getting absolutely obliterated and acting like fools, in the end they spill more than they drink, thankfully they usually aren't in till late the following day.
To avoid the drunken fools, stay away from Lowenbrau and Hofbrau on the first day and late in the afternoons. Better beer than that in other tents anyway.
