Lately I've been lurking on the #CSS channel on the Freenode IRC server and I've noticed a lot of questions that basically boil down to a developer not understanding the CSS box model.
The most basic things to remember are, in my opinion, the following.
There are block and inline elements. In simple terms, block elements get a newline after them. Elements like div and p are block elements. Inline elements usually reside in a block element; think of them as stuff you'd put in a paragraph. Anchors and markup like em are all inline elements.
Then there is floating. If you let elements float left or right, they'll "float" to the top of the containing element. Floating elements must have width applied to them. Multiple elements will float nicely besides eachother if there's room.
When you've got a problem, think about the above two things and see if you can figure out what's happening. Don't just ask in the #css channel "what's happening", try to come up with a theory and state it. It'll help you a great deal more.
Also read Mike Hall's primer on CSS Positioning.