You need to stay awake and evolve VERY fast.
Some marketing principles might stay valid but the online techniques you use can change very fast.
Why?
Well, take search engines for instance.
If they tweak their algorithm, what was a successful SEO campaign can literally turn into a marketing disaster.
Gmail, Hotmail or Yahoo can suddenly decide to label your emails as spam which means that only 5% of those who were previously getting your ads suddenly see them.
Or take the pay per click ads: 5 years ago, price per click was still an average $0.10 now, it's more like $0.80!
You realize what this means for your business, right? It means that you need to have a conversion rate which is 8x higher to achieve the exact same return on investment.
These are massive shifts and they happen fast.
Online, dozens of small changes like the ones I describe can hit the market and bring companies out of business in no time.
Your audience's needs changes as well very fast.
10 years ago people relied on email for their source of information.
Today, it's social networks.
The conclusion is that you must stay alert and evolve FAST with the market if you don't want to sink.