Shake it for several minutes a couple hours before you need it, then shake it for several minutes before use, and a few shakes every minute during use.
I rarely get clogs, and run my cans of paint out to spitting empty (I could eek a little more out, but it won't lay right)
The above advice is also good.
If it plugs up unmercifully drop the nozzle in a bowl of thinner for a few minutes.
Phil