Someone asked a question about this so I wanted to share how I do it:
Design my app/site/etc in Sketch.
Go to the export panel and draw my slices on the elements I want to export to AE.
2a. If I plan to separate things really granularly, I'll create a "background" and then I'll individually slice all the rows, icons or whatever I'm going to animate on a smaller level.
I take elements directly from Sketch into AE by dragging them into the project panel. I drop the larger groups in from Finder after exporting from Sketch. Screenshot: http://d.pr/i/AtUB
Once in AE, it's just like it is in Photoshop. Animate away! http://d.pr/i/WnKJ
AfterEffects has a pretty forgiving import flow which makes this all super easy.