If your H.264 encoded video does not start playback until the entire file has been loaded into the browser cache, first check the export encoding options in the application you are using to see if there is a "Streaming H.264", "Fast Internet Start", or something similar. Check / turn on if currently off, export a new video, and try it again.
If that doesn't help, then you may have encoded the video using an encoder or video editor that places the H.264 metadata at the end of the file (when Flash expects it to be at the beginning). There's an AIR app called "QT Index Swapper" that can fix this in an already encoded video. You can download the utility here:
1) Unpack the ZIP.
2) Double click on the .air document inside to install the utility.
3) Launch QTIndexSwapper and follow the directions it displays.
(Note: We are providing our own copy of QTIndexSwapper because the original version can't be located online anymore. Please contact us if there's an update on this.)