I have been contemplating how to get good sound on my videos. My first plan was to get a 4 channel mixer, a Shure headset mic (or a lapel mic). Then mic my amp with an SM57, send it to the mixer; use the headset/lapel mic and send it to the mixer; then I'll mix to taste, and run the output from the mixer into the audio input on the camera.
Well, that idea sucked. No way of adjust levels between voice and guitar, and the cheap mixer added extra noise.
Then, the light bulb came on. I did a test last night, where I used a lapel mic for talking, and an SM57 on the guitar amp. Both mics went into my old Tascam US-122 audio interface, and then I recorded these 2 tracks into GarageBand while the camera was running. I then mixed the 2 tracks in GarageBand, basically just panning them a bit left and right. It made it a lot better. The lapel mic picks up a lot of the guitar amp sound, but I don't think that is really hurting anything. When the 2 tracks are together, it still sounds quite good. I exported the mix as a lossless AIF file.
After I was done filming, I imported the video to the computer. I deleted the audio from the video in Final Cut Express (the audio that the video camera captured). I imported the AIF audio file I had exported in GB, and now I had to sync up the sound to match the video. It wasn't that hard really. Just a matter of finding the beginning, and then fine tuning it one frame at a time with the arrow keys on the keyboard.
Heureka! My upcoming videos will sound a lot better, guys!

The end result - a huge improvement in overall sound!
I am going to use this method from now and, and when I have the first video up using this technique, I'll be interested to hear what you guys think.