Skeletal Meshes Pipeline in Blender

Requirements:
- Must be a single mesh (Though you can combine several together before rigging)
- All Transforms must be zeroed out in Object Mode (feel free to align and resize the mesh in edit mode though)
- There must be a single root bone at the origin.
- There must be a minimum of three bones.
- All bones must be within a hierarchy (even, if only to the root - and all bones not within the hierarchy should be parented to the root)
- There must be at least one material applied. (unreal uses the material slots, but won't load the mesh with the material already applied: there's no point - it removes all of the fun of the material editor!)

The Unreal Exporter (Preferences>Add-ons> Input-Output>Unreal .PSK/.PSA) elsewhere known as ActorX is touchy: with Blender 2.63 I have experienced great difficulty in exporting a mesh due to what I can only guess as problems with excessive poles in the mesh (I'm a stickler for quad modelling, so there are a number of places where there just have to be poles in order for the mesh to flow in an agreeable way). This I  have surmised after spending an excessive amount of time cutting holes in my mesh before export to try to ascertain the root of the problem. In the end I OBJ'd the mesh to an earlier portable version of Blender (I believe it was 2.57) and found there to be no problems whatsoever exporting from there.

The principle advantage of ActorX, though, aside from the warnings which Unreal presents when uploading an older FBX export, is that all of the problems are logged at export time: you won't end up with a file which fails to load in UDK.