Is there anything special to pay attention to when preparing a bicycle for powdercoating? I am thinking of having an old bike powdercoated to spruce it up a bit.

What do I need to do to prepare the frame? Remove all components? What about stickers and decals? Some kind of chemical etching dip?

It is an aluminium MTB frame.
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.