I've gotta get some advice on buying a mattress.

I've chosen to live very simply over the last few years, with few regrets. One of the regrets I do have is that I wake up every morning with back pain largely because of the cheap foam mattress I sleep on. It's basically like a piece of 6" yellow foam on top of wood slats. I realize that even having that makes me quite wealthy and fortunate by local standards, but living with daily discomfort still stinks.

So I'll be headed back to the US in a little over a year for a furlough and with that move will come a relocation allowance. I'll need to purchase a mattress and won't have an extravagant budget, but I'm realizing increasingly that a decent mattress is a good investment. One more note: I'm 6' 3" (190 cm) and weight about 220 lbs (100 kg or 15.7 stone). I've had problems with every mattress I've ever owned gradually sagging and forming a valley where I sleep (even when I rotate and flip it.) Sometimes that doesn't happen until I've owned it for a year or more, so I'm hoping to not be disappointed again. Rarely happens for my wife's side, or not nearly as badly, so there's definitely a weight where many mattresses just throw in the towel and gives up. I'd like some recommendations on:

1. Mattresses that you've specifically found to be good for dealing with back soreness (for me it's in the lumbar region).

2. Mattresses that have held up well over time for people who are big.

I'm hoping to get some real value and satisfaction out of this mattress, since I'm likely not going to be able to afford another one for quite some time. I realize that a year in advance is a long time to start shopping in advance, but if I can get some kind of idea of what I want, then I can start checking out prices and budgeting appropriately.

Thanks so much!