The worst way to put space between paragraphs is to insert an extra carriage return. (See carriage returns for why.)
As with first-line indents, you want the space to be large enough to be easily noticed, but not so large that the paragraphs seem disconnected. A space equal to 50–100% of the body text size will usually suffice. The larger the point size, the more space you’ll need between paragraphs to make a visible difference.
Is space before a paragraph equivalent to space after? Sometimes. In word processors, the space between two paragraphs is the larger of the space after the first paragraph and the space before the second paragraph. Thus, if every paragraph has 12 points of space after, you’ll get 12 points of space between each pair. But if each paragraph has 6 points of space before and 6 points after, the space between will only be 6 points. To avoid surprises, I prefer to rely on space after, and use space before in special circumstances. For instance, a block quotation may need space before and after to look vertically aligned.