i tried several methods but this was the best:
cat *.MPG | ffmpeg -i - -b 1500k -ar 44100 together.flv
the * goes in ordered state so if you have videos from your camera they are already ordered.
I had some problems screencasting my desktop, i used the freeware from Copernicus … works great but is using the cpu on my 2.16 completely, i have two g of ram and the option: record into ram dosen’t work at all and i can’t record audio ;( i use the mpeg4 output type with h264. this output file is not convertable with ffmpeg: “no media or nothing found”.
Now I use Screen Mimic which is good but uses strange flv output formats … the mov output is good and
fast. Now i am converting those mov videos with ffmpeg like this:
ffmpeg -i somemov.mov -sameq -g 500 someflash.flv
I had the best quality experience with the -sameq option and setting the group of picture size to like 500 gave me additional “compression” 
i think the -b video bitrate option is not needed anymore because of -sameq. I had a 26second video screening my surfing on lots of webpages with many images. the mov file was 50mb and the resolution was 1280×800. after converting the quality stayed the SAME and the file was only 6mb big!!!!
history:
I also tried Quicktime Pro -> didn’t work somehow
somehow copernicus speeded up the video by 300% but this was solvable with premiere which gave me horrible output quality, after effects then did the job ok but really really really slow
