I read a piece by a very well known internet marketer (won't name him) who suggests that the best way to keep ranking is to buy carefully selected links in not too great a number on related sites. He also says there will be people on various sites and forums who pass themselves as 'experts' telling you that it is a path to disaster, but what they don't tell you is that they indulge in these practices themselves even though they tell others not to. This discourages their competitors from getting links to their sites while they improve their own rankings.
I don't know what to make of that.
I did use to buy a handful of carefully selected links and few article directory submissions each month as well as get some natural links (around 20%) and I always use to find I use to keep getting a small a boost in the rankings.However, for the last year I have completely stopped buying any links whatsoever, and while I still get a few natural links here and there, I just feel I have stopped making progress traffic wise despite adding content on a regular basis. The traffic increase nowadays seems only proportional to the content I am adding rather than getting a boost in the SERPS.
When I first started out in web sites it use to be relatively easy to gain natural links from other sites without the need to solicit, but people just seem much more reluctant to link to other sites nowadays. The sites people seem to link are ones that are big names, usually run by big media companies. These sites have huge brands and people don't think twice before linking to them but have now started to overlook the regular sites (even if they are semi-authority).