There was a time when peeing on my mulch was my low-tech waterless urinal. But since I moved to the city, abandoning my rural green (elitist?) dream, I've had to be a little more discreet about when and where I "peecycle".
After all, however much I care about water conservation, I'm reluctant to get myself arrested for returning nutrients to the soil. Still, we urban dwellers do still have options.
Here are some ideas:
The selective flush and shared flush are low-risk ways to conserve water and avoid embarrassment with your neighbors. (Unless they come over for tea unannounced.) They don't do anything about actually returning nutrients to the land, but they do reduce the water footprint involved in flushing.
Carefully siting your compost bin
Even the smallest urban yard often has some nooks and crannies that are hard for neighbors to see into. Behind the garden shed, and tucked up against a privacy fence, it's possible you can create a compost heap that's perfect for the occasional "deposit". Of course, pee too often and you may have smell issues—but otherwise, this is a pretty good way to go, at least for us blokes. (It turns out that male pee may be better for compost anyway...)
A watering can
My neighbors don't seem the types to read TreeHugger (amazingly, there are some people who still don't)—so I'll go ahead and confess. While my out-of-sight compost heap provides occasional relief, I have also discovered that a watering can can be a useful tool. Pop inside. Do what you need to do. Water it down. And you can then add it to your compost or even use it directly in your garden.
A high-tech toilet
Of course you (or your city) could always invest in some more extensive (and expensive) nutrient-recycling infrastructure. But be-warned, there's some hot controversy on pee-cycling versus composting toilets, so read up before you decide which way to, ahem, go.