Let’s start with one of the most important aspects of Flash building.

Reuse your symbols

Where possible you should reuse symbols instead of creating new ones.
I.E. you need a cloud graphic, an oval button and a spot light effect. For each of those elements you only need one base symbol. Start by creating a circle of any color then convert it to a graphic symbol. Drag an instance of the circle from the library to the stage, go to the properties and set the circle color to white via the ‘tint’ option. Copy and paste this circle a number of times, changing size, shape and opacity where needed to create your cloud. Return to the stage and create your other two elements using the original circle symbol following the same color/alpha change process to get the desired effects.
The same types of symbol reuse can be used with a number of different shapes or combination of shapes. Need a forest? Create one trunk and one leaf, copy paste the leaf to make the tree, return to the stage and pull in as many trees as you need from the library.

Symbol reuse in this manor helps Flash keep the number of individually rendered elements to a minimum.

Keep symbol numbers to a minimum

The number of individual symbols you create works hand in hand with symbol reuse. The simple rule is: do not create symbols unless you need them and where possible create a new symbol from an existing one and transform it.

Never animate using keyframes

Using separate keyframes containing separate artwork is a sure fire way to make your file huge. As a rule, you should always animate your symbols using motion tweens, one moving element is much smaller than using multiple elements to create the same effect.

Never use shape tweens

Under no circumstances should you be using any type of shape tween. The shape tween feature will bloat your movies and substantially slow down play back.

Use Jpeg’s sparingly

While Jpeg’s are sometimes needed to complete a Flash project you should try and limit their use to critical applications only. Jpeg’s affect the file size of Flash movies in a big way, if you have to use them, make sure you’ve used the ‘save for web feature’ in Photoshop before importing them into your Flash library.

Stream user initiated audio and video files

Audio and video files should always been linked to and loaded as a stream from your main Flash file, never include these files inside your main file.
To ensure optimum User Experience, audio and video files which are loaded from within your Movie should always be initiated by the user and should contain Play/Pause/Stop and/or Mute functionality as a minimum. In some cases the ‘no auto playback’ rule may be broken; Video files inside a Flash ad may start immediately, audio files should still require user initiation.

Depending on the type of Flash file you are creating, some or all of the following tips may help you reduce the size of your exported Movie.

Limit your color palette

Using a limited color palette, much like saving a GIF at 128 rather than 256 colors, can substantially lower your Movie size.  It is recommended you use tints of existing colors where possible.

Limit use of gradients

Limiting the use of gradients works hand in hand with limiting color palettes. A good rule is: two is great, five is too many. Remember this refers to individual different gradients, reusing the same gradient is fine (see: symbol reuse).

Limit use of transparency

Using transparency can allow you to create some great effects, but those same effects are slowing and bloating your Movies. As a general rule: individual items which include transparency, I.E. a button with drop shadow, are fine. You should try to avoid layered transparent items and you should definitely avoid tweening multiple transparent items across each other as this will result in playback slowdown.

Should you need to further shrink your Flash file after following all the tips above you may wish to try the following;

Save for an older version

Unless you absolutely need your Flash file to play in the latest plugin it is recommended you save for files for Flash Player 8. Saving files down from a later version to Flash 8 can sometimes shave as much as 25% off the file size.

Drop frame rate

25 FPS+ is the realm of TV and video, your Flash files should be around 15 FPS for optimum playback/file size. Dropping the frame rate, even below 15 FPS may be needed to lower your file size further.

And that’s about all I’ve got in the ‘must haves’ category, if you think of anymore ‘must haves’ or just want to share some advanced techniques please feel free to do so in the comments.