I don't edit CSS very often, and almost every time I need to go and google the CSS box model to check whether padding is inside the border and margin outside, or vice versa. (Just checked again and padding is inside).
Does anyone have a good way of remembering this? A little mnemonic, a good explanation as to why the names are that way round ...
Print the diagram from the Box Dimensions section of the specification, and put it on the wall.
To me, "padding" just sounds more inner than "margin". Perhaps thinking about the printed page would help? Margins are areas on the far outside - generally, you cannot even print to the edge - they are unmarkable. Within those margins, the content could be padded to provide an extra barrier between the content and the margin?
Once you work in CSS enough, it'll become second nature to remember this.
When working with CSS finally drives you mad the padded cell that they will put you in has the padding on the inside of the walls.
I've just learnt it over time - the box model is fairly simple but the main reason people find it hard is because body doesn't visibly break the model.
Really, if you give body a margin and a background you should see it surrounded by a white strip. However this isn't the case - body's padding is the same as margin. This establishes a few incorrect things about the box model.
I usually think about it like this:
Create yourself a commented base stylesheet which you can use as a template whenever you need to create a new site or edit an existing site.
You can add to it as you grow in knowledge and apply it to various different browsers to see how various things behave.
You'll also be able to add in comments or examples about other hard to remember stuff or stuff that is counter intuitive.
Add border, even just temporarily. As you play with the numbers, you'll see the difference.
In fact, temporary borders around elements is a helpful way to work, such that you can see why floats are dropping, etc.
I know this is an answer to your question, but more of a tip. Whenever I am dealing with margin and padding, I will add a border around the part you are working with, then from there, it shows me the room I have to work with. When I am all set, I remove the border.
You are using a box. If you were putting something in a box you would put some padding inside to make sure it didn't smack against the sides. Margin would then be the other thing.
Tim Saunders gave some excellent advice - when I first got started with CSS, I made a point of building a good, fully commented base stylesheet. That stylesheet has changed many times and remains a terrific resource.
However, when I ran into my own box model problems, I started using 'Mo Pi'. As in, "I'm not fat enough, I need to eat mo pi." Strange, but it worked for me. Of course, I put on twenty pounds while learning CSS...;-)