Directions to the Empire Polo Fields on the offical website are ideal to get there...if there weren't 60,000 doing exactly the same thing at exactly the same time. We took this way day 1 and took over 2 hours to get to the parking lot. (keep in mind that its really only 10 miles from the off ramp). Search websites, forums, etc to find alternate routes. I can't remember exactly the way we took, but day 2 we were in the parking lot in under 1/2 hour. Ask people while you are there also...they know the area of course.
Luggage and bags:
Definitely a good idea to bring a backpack or large purse of some type for all your supplies.
Clothing/Shoes/Weather Gear: No need for sweatshirts...you can buy band merch as you walk in if you feel you might be cold. But it stayed in the low 80's at night when I went (coachella 2006). I was told to bring a sweatshirt and I lugged it around all day.
I brought a hat, and glad I did! No need for huge cowboy hats...just something that covers your face from the intense sun.
Shoes- Whatever you can stand to walk in for 12+ hours. Again I was told to wear tennis shoes and hated every minute of it. The ground is really nice grass once you get inside, although dusty/sandy on the way in. I wore flip flops the 2nd day with no problem.
Toiletries and Medical Supplies: Sunscreen!!! Everything else you will find at the medical tents w/ no problem (bandaids and such)
Photo Equipment: bring in anything that doesn't have a removeable lens.
Camping/Beach/Outdoor Gear: I didn't camp...so not sure!