Getting better support than Slic3r generates
I know many Slic3r users - myself included - add support material to the model itself before importing it into Slic3r. 

I personally favour MeshMixer for support generation, as the supports are much more predictable and easily removable. In complicated cases I also add supports in my CAD software. 

Although a not free, the support generation in Simplify3D is supposed to be great, allowing for custom placement.

I also found this review that compares support settings in Meshmixer, Slic3r and Cura fairly well. Here is the summary:


  Meshmixer
  
  This support was the most efficient in material use however it required the most manual tweaking to print properly. In addition the marks it left once removed were more noticeable than Cura and (sometimes) Slic3r. While the settings could probably be further modified to improve the performance this support type appears the most limited for future improvements.
  
  Slic3r
  
  The support's performance was variable - by far the best in some situations (fox's head and tale) however the worst to remove with the most obvious marks in other areas of the same model. This may be down to my chosen settings and with some more tweaking I may get better results. The most obvious general flaw is that it does not leave a big enough gap between the support and the unsupported areas of the model (like the foxes legs or the lower roots of the planter)
  
  Cura
  
  While some of the support left marks, overall it was the easiest to generate support which performed consistently well. Once again though slight tweaks could improve this further for specific models. 
  
  Overall Cura wins my "no time to tweak - got to make it work now" award.