Eliminate unnecessary rules (lines) between columns and rows.
Don’t repeat information. If some expression (such as units) occurs in every cell in a column or row, move it to the header.
Don’t use a table if a plot would be better. If you have a large amount of information and readers are not interested in exact values, it is often better to display it graphically.
Round aggressively. Readers rarely need more than one decimal point.
Consider rearranging or transposing the table. If readers are likely to compare two quantities, place them in adjacent columns (it is slightly easier to compare elements between columns than between rows).
Include a reference to the table in the text of the document and highlight important features.