I don't know if this is the best way to do it but I use width with margin-left and margin-right set to auto and haven't noticed any problems yet.
Code:
blockquote, div.block {
margin-left: auto;
margin-right: auto;
margin-top: 1em;
width: 85%;
}
The 1em space below it is in the p tag:
Code:
blockquote + p, div.block + p {
margin-top: 1em;
}
I heard it was better to prefer margin-top over margin-bottom.