It may be plain obvious that you want to keep bandwidth down to make your pages and images load quicker, and to save you a bundle, and if your site is quite busy, and you're working with a sliced layout, I have some tips for you lucky people out there NOTE: bandwidth is sometimes referred to B/W.

1, Optimize ALL your images.
Yes, that means every single one of them. On a layout, this is most importent. I use Macromedia Fireworks, and I initialy slice at the highest quality image format avalible. When in Dreamweaver, I always click the 'optimize in Fireworks' button. By doing this from Dreamweaver, It'll also fix the <img src="blah.jpg"> tags.

2, Optimize your HTML.
This is where it gets a little complicated. You can either do this by hand, or by using HTML Shrinker light. To do this by yourself, you will need to get all your code onto one line, with no spaces inbetween any tags. The only spaces you must miss out, is the spaces in the actuall text of the website. I hate this job, so I let HTML shinker do it for me

3, Thumbnail images.
For large images (wether it be large in pixels or large in bytes), always provide a thumbnail. The viewer of the thumbnail will decide if he wants to view a large picture or not, saving you B/W.

4, DON'T EVER have backround music.
They take up WAY to much space, bandwidth, and piss the viewer off if they are listening to, ohh, say, System of a Down.

5, External CSS (thanks to greg)
Use external stylesheets or scripts when possible. For example, if you write your CSS in each page, it will have to load on the visitor's computer every time he visits a different page. When you use an external CSS file, however, it will only have to load once.

6, HTML and images (thanks to Jaymz)
Never use images when you can use HTML or CSS to do the same thing.

7, Reuse, reduce (thanks to Jaymz)
Reuse repeating sections when possible, for example, if all of your content boxes are the same, use the same background image.

8, Text>Images (thanks to Jaymz)
Never use images to replace text. This eats bandwidth and is completely unnecessary.

9, External Javascript (thanks to Jaymz)
If you use javascript, link to an external .js file instead of putting the javascript in the head of every document.

10, Hotlinking
Urge your veiwers to upload link buttons, banners, pictures, etc, to their own server. This will save A LOT of bandwidth if loads of people want to use them on their site.

Great tips, but here's one more:

5. Use external stylesheets or scripts when possible. For example, if you write your CSS in each page, it will have to load on the visitor's computer every time he visits a different page. When you use an external CSS file, however, it will only have to load once.

Added Thanks for that.

6. Never use images when you can use HTML or CSS to do the same thing.
7. Reuse repeating sections when possible, for example, if all of your content boxes are the same, use the same background image.
8. Never use images to replace text. This eats bandwidth and is completely unnecessary.
9. If you use javascript, link to an external .js file instead of putting the javascript in the head of every document.

You lucky thing. Your tips are added.
Oh, and I added another.

You forgot to turn your PC off easiest way to safe bandwidth

lol @ Donna! That's ISP bandwidth I get an infinite amount

great tips! Very usefull