This is a technique I've been trying to work on myself. I referred to Nobuya Sugawa's technique book, which unfortunately is in Japanese. I had a friend who knows Japanese help me translate parts of it. Anyhow, in it, I can't remember exactly, as I've only seen it once, it says something about playing a note, such as a palm key D, and then try to use your voicing, tongue, and embouchure placement to get it to sound a C#, then try playing the D fingering, and C# fingering, alternately, but trying to always sound a C#. I know this probably sounds somewhat confusing, I'm not the greatest at explaining. However, then try doing it fingering a D and sounding a C, alternate, and keep going down. Try it with different notes, and then eventually, you can get a feel of what the feeling of portamento is like. Then, I tried doing intervals. For instance, I usually started with a G->A in the low register, for some reason worked for me. I'd try to manipulate the 3 controls I talked about above to achieve a slide between the notes. Next I went G->Bb, then G-B, and continued the pattern chromatically all the way as high as I could go. (Including altissimo). I'm still not the greatest at portamento, but I hope this might help for you, it did for me.