You have a few options available. There are a few racks out there that will work with many different full suspension systems. Some Old Man Mountain racks will work with some FS bikes as an example. Some riders use seat post mounted racks for lighter loads (10 pounds). Others will skip the racks and panniers all together and use frame, seat and bar bags.
Just as an example, we bumped into a guy on this bike on the Colorado Trail. This guy was insane light with nothing more than this frame bag and a very small pack. We found him on day 3 of his ride.
That milk crate idea might be not so super unless all you plan to do is ride gravel roads. You sure won't be riding any singletrack with that set up. However, If you use a frame bag, seat bag, bar bag and pack, you can easily haul 30+ pounds which would give you a good 3-5+ day range with available water. Adding racks to the rear would allow you to pack a bit more. That extra volume would give you more room for water, chow and maybe more clothes to ride through more severe weather.
This all really depends on what sort of ride quality you want to retain. It is possible to fit your bike with 3-5 days worth of kit and retain much, if not nearly all, of the lively ride quality your bike has now. Or, you can sacrifice some, or all, of that snappy ride quality and go with racks, bigger bags and more stuff. For jaunts under a week, I have a hard time convincing myself to use racks and panniers.
But, if you're prone to haul a bit more gear than most, those panniers might be a must. Personally, I would still use a frame bag. Riding with a frame bag is pretty amazing. The weight vanishes in that location.
Check out Old Man Mountain racks:
http://www.oldmanmountain.com/
For a neat new seat post rack:
http://www.arkel-od.com
Another option for rear racks has been around only a short time:
http://www.freeload.co.nz
Lots of options.
Lastly, there's always the Po'man's set up. A stuff sack lashed to the seat. A stuff sack lashed to the bars and a backpack. With the right gear, that'll get you at least on a good long overnighter. Below is a pic of my rig. On the bars I have a $15 Outdoor Research "Hydrosac" stuff sack held to my bars with webbing straps. On the seat is another stuff sack mounted to a $19 inverted Blackburn water bottle cage mount. Darn cheap. My sleeping bag is under the seat. The bar bag held a Big Agnes sleeping pad and misc gear.
Backpacking gear is getting so insanely light that there's little need to exceed 35 pounds of kit. 35 pounds does not usually warrant a rack, panniers and a rack bag.
I think this set up is pure genius:
That set up made to fit around full suspension linkage, and paired with a 30 liter pack like a Camelback Cloud Walker would easily give you 35 pounds of capacity. It would also make your bike ride beautifully without any of the cumbersome heft that haunts bikes with panniers. That bar bag would easily accommodate a small tent.
That set up is so awesome. A full week of ripping singletrack would be no issue with that rig.
Those bags (same as those I have) are made by:
http://carouseldesignworks.com/main.html
