My printed parts consist rafts, supports and other extraneous filament when printing with ABS or PLA.
What are efficient general techniques of removing them?
The best way to get rid of them is to change the design of the printed object to make them unnecessary.
Instead of printing the one part with support material, the piece can be split into two or more parts which can be printed without support material and assembled after the printing.
Given that this is not always fully possible, a convenient way to get rid of additional structures is to use a different fillament for them that can be removed easily. This list of printing materials includes Polyvinyl Acetate (PVA), which is water soluble. You can wash the support material away given that your actual printign material is not water soluble. Here's a quote from the website (emphasize mine):
PVA (Polyvinyl Acetate) filament prints translucent with a slightly yellow tint and is primarily used as a 3D printing support material because it is water-soluble, meaning that it will dissolve when exposed to water (and so MUST be kept dry prior to use). PVA is most often used with 3D printers capable of dual extrusion: one extruder printing a primary material (such as ABS or PLA) and the other printing this dissolvable filament to provide support for overhanging features. PVA 3D printer filament is available in 1.75mm and 3mm.
Increasing the distance between the support/raft and the print should allow for easier removal. Some slicers (such as Slic3r) have settings for 0.0 mm spacing for water dissoluble supports, and 0.2 mm for regular plastic supports.
Changing the upper surface of the support also can help. Some slicers will leave the support as lines all the way to contact with the print. Others will put a full layer on the top of the support before the print. The full layer allows for better adhesion and the print not falling between the lines, but it makes it harder to remove.
When the raft/support is done with lines, putting pressure on the lines in the opposite direction should cause them to break off. As you break some of the outer ones, the support can be pulled down which will continuously rip the support off. There may be a few pieces that remain. These can be removed with wood chisels. Sliding the pointed end between the support and the print will cut into the plastic which removes the support. Note: always push the chisel away from you as it may slip. You do not want a sharp blade moving uncontrollably towards your body./