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. Between 4 and 10 points of space (0.06–0.14″) 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.
Space between paragraphs is a poor choice for court filings with line numbers. The only way to preserve the vertical alignment is to make the space between paragraphs equal to a whole line space. This leaves a lot of big gaps in the page and eats up your page limits. It can work with headings, however, because a document contains fewer of them.