Stiffening, cage if you feel froggy, proper bumpstops, and well tuned shocks.
That being said, you could just buy a $1000 beater and flog the shit out of it. I did that to my 98 XJ for something like a year, it's been bottomed out on the shocks so many times the bushings came out in pieces, the front pinion/driveshaft chewed a hole in the exhaust, and the steering linkages scratched up the oil pan, and I drive it 140 miles a day for work with no problems. I paid $1000 for it in 2010 and I think I've put something like 700 in junkyard parts on it now to make it look nicer and keep it going.
Stock except for a V8 ZJ tie rod, helper shocks in the back to deal with the saggy leafs, broken-off rear bumpstops, and one front shock (240k on it plus my abuse, so I can't really complain) is completely, utterly dead at this point. Oh and I put a grand caravan radio in it, which helps the suspension a lot.
hell, XJLI has ridden in it while I did some of this, I came out of that run with scratches and scores all over the bottom of the gas tank and then drove it home.
Worked great for me, and I've been able to steadily upgrade until now I've got a pretty nice XJ, so I'm hoping to make this one last a while. 
