Here's a great article with tips on tuning your guitar using harmonics courtesy of the Fender Website.
Absent an electronic tuner, you know how you can get your instrument to a point where it’s at least in tune with itself using the harmonics at the fifth and seventh frets? And you know how that works on guitar for every string pair except the G and B strings? If you don’t, here’s how it works: Sounding the fifth-fret harmonic of a string and matching the seventh-fret harmonic of the string directly above to it is one of the easiest and most reliable ways to tune a guitar.
So if you sound the fifth-fret harmonic on your low E string, you can tune the A string (the string directly above the low E) by sounding its seventh-fret harmonic while the fifth-fret E-string harmonic is still ringing. The notes should be identical. If the two strings are even slightly out of tune with each other, you’ll notice an audible oscillation or pulsating effect while both notes are ringing together. You’d then adjust the A string until the oscillation goes away and both harmonics sound exactly the same.
You can continue in this fashion for the A and D strings, the D and G strings, and the B and high E strings. This method works because all these string pairs are tuned to a musical interval of a fourth. Notice, however, that this skips one pair of strings—the G and the B. You can’t tune the B string to the G string using fifth- and seventh-fret harmonics because that’s the only string pair tuned to a musical interval of a major third (the reason for that is a whole other story). Many assume that in that case, the G/B string pair can’t be tuned using harmonics.
Not so. Here’s the little secret: You can use harmonics to tune the B string to the G string—by sounding the ninth-fret harmonic of the G string and tuning the fifth-fret harmonic of the B string to it. The ninth-fret harmonic is a little more delicate to sound, but it’s definitely there, and tuning the B string this way is as absolutely reliable as harmonic tuning for all the other string pairs.