For getting there from the north, I'd recommend considering taking I-10 east out of LA and getting off at Indio, then hitching down the 111 to Niland (you'll have to walk a few miles). Last time I did this, it took a couple hours to get a ride down the 111, but just about anyody going to Slab City will pick you up, and people are always going there. At Niland, assuming you didn't get a right straight to the slabs, any local will point the way, and if you start walking there (it's only a few miles out) almost anybody driving out there will pick you up.
If you go all the way down to San Diego to hitch the 8 to El Centro then back up, it's kind of a roundabout way of getting there, and I've found I-10 to be generally easier to hitch than I-8 (lots more traffic). For hitching east out of LA on the I-10, check out this thread:
http://squattheplanet.com/getting-there/hitchhiking/4741-need-socal-hitching-info-ask-me.html
Oh, and like MURT said, get food stamps if you can, or at least bring enough food to last you a few days. Hit up a food bank or something. And I can't emphasize enough: WATER. It is freakin' HOT out there right now (100+ degrees at this point of time, and don't expect that to change much), and the nearest source of water will be Niland, which is a few miles away (long walk without water). I'd bring at least three gallons, one for each day. And much as I hate sunscreen, I'd recommend it unless you want to leave Slab City red as a lobster. Know the signs of heat stroke and hyperthermia, and how to avoid both.
Remember folks, you're going to be in the desert, and while the rest of us won't let you die, we shouldn't have to be responsible for keeping your ass alive.