any secret to keeping them alive? They almost always get nullified in one turn
The 'trick' there is that your enemy needs to be under enough pressure to make keeping a healthy pool of spies impossible. If your economy is far superior, you can probably spend your way through decimating their store of spies and stay ahead in producing new ones. Getting them involved in a war with someone else can also help.
I tend to stockpile spies until I'm very serious about a war, and then I try to blanket every econ building I can find. Even if I lose a bunch from that first wave, as long as a good number make it through, the enemy will have a harder time paying for new spies.