I have noticed in your pastebin that you have mis-spelled content in the CSS. Also you have put a ‘+’ to seperate the two selectors, when with CSS syntax you should use a comma (I think?) try and make them changes and see what you get…
Okay, so there are a couple of things to consider with this. The first is that the direct sibling of the h1 is actually the div, so the sibling selector won’t work in this case. I get caught out a lot with this, but I have try not to use sibling or child selectors if I can avoid it, as targeting via specific classes is much easier to work with.
Secondly, you should always try and work with your margins in a single direction only (I generally always use margin-bottom). Here is an example of what I’m talking about: http://jsfiddle.net/joshnh/ZBgYA/
If you wanted to now remove the margin between the h1 and the p, you could easily do so. Also, you will find that it is much easier to move elements around if the margin is always in one direction. Your code becomes much more modular.