Is there anything special to pay attention to when preparing a bicycle for powdercoating?
You'll definitely need to remove all components.  For threaded bits you have two choices - leave junk bolts in there that will get powdercoated over and then carefully remove them (leaving a bit of an edge around the bolt hole) or let them get masked but almost certainly somewhat powdercoated into and use a tap and die set to chase them before reassembly.  Be prepared to face/ream/chase the headtube and bottom bracket in any case.  

Do make sure you've cleaned it as much as possible, removing any residual grease and road dirt.  I wouldn't worry about the stickers and decals, the media blasting to prepare the frame should take care of that.  With the relatively low cost of media blasting I wouldn't bother trying to strip the paint myself.

Make sure you find a powdercoater that is familiar with bicycle frames; particularly when it comes to media blasting they need to understand how thin the tubing is.