Pskim's description of octave gliss. is the best.
Anyone reading this thread should be advised to take suggestions about how to play octave glissandos from someone who actually played them in the context of a piece of music - i.e. pskim, who has played the Waldstein.
The only thing I would add are two suggestions. 1) you can "work up" to an octave gliss by first practicing a smaller interval, a fifth or sixth for ex. After that interval is comfortable, try a larger interval 6th, 7th, and finally octave. You will notice that your 5th finger flattens out more as you spread out to an octave.
2) Don't try to play it to the bottom of the key bed. Try to find the point where the note sounds, and use that point as the "level" for the gliss.
Finally, to correct youngvirtuoso about "regular glissandos" - I assume he means single note gliss. - you do not always play them with your thumb. The rule is as follows: if the gliss moves away from your body: ie right hand starting on middle C and moving higher, or left hand starting on middle C and moving lower - you turn your hand over and play the gliss with the nail(s) of your third and second fingers - if you can control it with just your third finger nail, that's fine, otherwise use your second and third finger nails.
For single note glissandos moving toward your body - right hand starting high and going towards middle C, or left hand starting low and moving toward middle C, for example - you use your thumb nail.