If you're working Twilight, after your sort is over, talk to the leads on Loadside and Unload, and ask if they need any help. If they don't, just split, but give them your number before you leave. If they want to use you, tell them you'll take your 30 minute break, and come back to work for them.
I've showed up to Preload early Sunday/Monday mornings because there's a lot of guys for that sort who call off, and knew they could use me. All it took was that one day for the Unload lead to see how I worked, and asked me if I wanted to come back the next day, and so on.
They're supposed to go by seniority when they use double-shifters, but they won't always make the effort to call the senior most hourly if they know you're the harder worker. I don't even bother putting my name on the double-shifting list anymore, because all the leads from Twilight and Preload know me and my work, and I get called throughout the year to double-shift, like I am now.
Just remember not to brag about it. There's always some worthless A-hole who has zero intention of doubling, but wants that "free" grievance check money. These are the guys that kill double-shifting for those that are actually willing to work when they're needed.