the thing is you really do have to monitor and follow whats going on..
things can change over night.. for instance just like the current google dance thats going on.. :)
for 1 site / a few terms its probably quite easy to do... but when you have alot to monitor/track/keep highly ranked.. it's a full time job.
it's all about keyword analysis and finding the right niche for you then implementation / continued monitoring. :D