Work with video in the native format of your project until your final output
Use raw footage or the least compressed footage that is available to you. Each time that you compress video using a lossy encoder, you reduce the quality of the video. Though one generation of quality loss is often acceptable, re-encoding and recompressing already compressed video can degrade the quality beyond what is acceptable. Also, video that has already been encoded and compressed may contain noise and artifacts that make the next encoding and compression step take more time or produce a larger file.
Make your video as short as possible
Trim the beginning and end of your video, and edit your video to remove any unnecessary content. See Crop and trim source before encoding.
Adjust your compression settings
If you compress footage and it looks great, try changing your settings to reduce the file size. Test your footage, and modify compression settings until you find the best setting possible for the video you are compressing. All video has varying attributes that affect compression and file size; each video needs its own setting for the best results. See Encoding and exporting.
Limit rapid movement
Limit movement if you are concerned about file size. Any movement increases file size. Shaky camera work, rolls, and zooms are particularly bad in this regard. You can use motion stabilization features in After Effects to remove extraneous movement.
Choose appropriate dimensions
Select an appropriate frame rate
See Frame rate.
Choose an appropriate number of key frames
See Key frames.
Reduce noise and grain
Noise and grain in source images increase the size of encoded files. Ideally, use utilities in Adobe Premiere Pro or After Effects to reduce noise and grain.
The same considerations exist for audio production as for video production. To achieve good audio compression, you must begin with an audio file that is free of distortion and audible artifacts introduced from the source recording.
If you are encoding material from a CD, try to record the file using direct digital transfer rather than the analog input of a sound card. The sound card introduces an unnecessary digital-to-analog and analog-to-digital conversion that can create noise in your transferred audio. Direct digital transfer tools are available for both Windows and Mac OS. If you must record from an analog source, use the highest quality sound card available.
If your source audio file is monaural (mono), it is recommended that you encode in mono for use with Flash. If you are encoding with Adobe Media Encoder, and using an encoding preset, be sure to check if the preset encodes in stereo or mono, and select mono if necessary.