Before anyone jumps in here and yells (or otherwise states) that HOLY CRAP IDIOT YOU SHOULD NEVER OVERCLACK YOULL BURN THE WORLD!!!11!ELEVEN, please realize that he did ask advice on how to safely do it, meaning that he's going to. Just thought I'd throw that in first since most threads like this have at least one naysayer that gets pissed off because someone else is OC'ing. It'll be ok.
/shortrant
The first thing I would do is google your particular CPU and see what others have achieved, and read what all they went through to do it. Lots of Asus boards like yours (the sabertooth is pretty high end) allow for "safe" overclocking, in a sort of "stepped" manner. In other words, sometimes it just knows right away when you open the OC utility that X speed is safe and gives you an option to do it. This is safer than manually editing values. In the case of my old machine, it ran 3.2 out of the box, and had a "safe step" that the board detected as 3.7. I hit go and it rebooted and said the changes had applied successfully. It also said that 3.9 might be possible. Tried it and it failed. Reverted to 3.7 and ran it for 3 years like that before I sold it last year to a guy who is still running it at 3.7 with no problems (AMD Phenom X6 1090T on a Asus Crosshair IV btw). If yours has anything like that available then thats the way I would do it. If you want more and want to get into manually OC'ing....just be very careful and read as much as you can about your particular chip AND the combination of it with your board and boards with the same chipset as yours.