Depends on the type of harmonics. I think that you are talking about natural harmonics, if so there are only a few places on the neck where they will sound clearly (known as harmonic nodes) - the 12th fret, 7th, 5th, 4th, 3.2, 2.8, 2.3, etc. The closer you get towards the bridge the harder they will get, generally they are easier on the thicker strings too. If you use you bridge humbucker with plenty of gain they should all sound clearly.
Another type of harmonics which you will hear a lot are pinch harmonics (listen to some Dimebag or Zak Wylde riffs). Getting a consistent sound here is harder and more complicated as notes are generally fretted down moving the nodes.