Standard suspension is designed to make the vehicle feel comfortable until it's out of the sale yard. Beyond that, it does a mediocre job of preventing the bump stops from being over-used while the car is on a hoist.
In truth, the stock suspension is designed to carry a light load - somewhere from 50 to 100kg settles the rear of the car nicely. Two bags of cement should do and if it doesn't work you can cut foot-shaped holes in the top, add water and the Nissan salesman's feet just before you go "fishing".
Dual or multi-rate leaf springs are definitely the way to go. My dual-rate Dakar (Old Man Emu) leaf springs are absolutely fantastic. We were barreling along Box Creek Road (south-western NSW north of Balranald) at 90+km/h with the caravan attached and it was smooth as.
Good shocks that dissipate heat quickly are essential. Thin, single-core shocks will overheat and wear a lot faster - something like the Old Man Emu Nitrocharger Sports is good (that's what I have) and the Tough Dog Foam Cells are reputed to be ok too.
Lowering the tyre pressure isn't a bad idea but you do have to be concerned about sideways pressure on the tyres, which could cause the beading to come loose. You can use bead lockers (from what I understand they're a great idea but they're illegal for some reason I can't fathom) or you can keep the pressure high enough to avoid the problem - around 25psi would be the minimum. Below that you have to take care when cornering, and hope that you don't hit a rock with the edge of the tyre.