go with friends ... if you forget something .. they are more than likely to have remembered it
the other thing to do is to group together with people who have been before and see what they take.
my first doof i took way too many clothes .. now i just take a shirt for each day, underwear, socks (though generally they are for night time use only... during the day you run around without shoes/socks ... well you should anyway.. its fun)..
re pants ... depending on where you are .. i take a pair of shorts and a pair of jeans. usually i'll be in these for the whole party. you could take a second pair in case something happens, eg you get covered in mud, water, juice, etc but hey this dries out fairly quickly too if left in the sun :D
take some food ... number of festivals have food to be bought .. but sometimes its nice to munch on some fish outta a can or two minute noodles or something
a jumper! nights at doofs can get cold coz its out in the country. sometimes 1 jumper isnt enough! depends on time of year and location
camera and spare batteries... must have for remembering the event .. just be sure not to drop it / cover it with mud, dust, dirt etc
if you're going to rainbow serpent, make sure you take a Hawaiian tshirt for the Luau on Sunday arvo ... they give out Lei's (those hawaiian necklaces made of flowers) and they play really funky music. its a must see and take part in
was there any questions you had in particular ?