Let's hear your most effective SEM tip.
Effective for PPC & besides Test, Test, Test
Make sure you align your keywords, ad text, and landing pages. Don't bid on keywords and send them to your homepage and expect great conversion rates. for most of us it takes more effort. You pretty much need a landing page for every adgroup...if possible.
There are lots of good blogs on PPC, but I really enjoy http://www.clickequations.com/blog/
You content needs to be great, but remember the rule is "Share, not Sell!"
Help others achieve what they need, and they'll refer you as a trustworthy source. That will send more visitors to you. Some of those visitors will take a look at what other things you have to offer, and some will eventually buy from you.
So, it all starts with great and useful content. Write it and get it online continuously and consistently (and optimize it following all the best practices).
One thing to keep in mind when optimizing your content is to use the keywords that people search for, not the keywords that you think you are about (an example I heard the other day was someone trying to optimize a website using the keyword "orthodontist". Most people can't even spell that word, so try something like "correcting misalignment of teeth" instead)
Hope this helps Michel