While graphs are long known to need care to make them effective, tables can use some love, too. The basic grid-like table with boxes between all cells is not the way to go.
Remove any vertical lines. Vertical lines can almost always be removed. A good table will only have three horizontal lines - two to frame the table above and below and one to separate the header row from the table data.
Add extra space between rows. This can more clearly separate data than, say, horizontal lines. If you really need a line beyond the main 3, consider a dashed line.
Consider cell alignment. When in doubt, align on the left. But often numbers should be aligned on the right. Having equal precision after a decimal point can make right-aligned numbers easier to compare across rows.
Have a caption which has the table number (e.g., Table 1) and that explains the data shown in the table. Table captions go above the table.
Light gray can be used to separate regions of a table, especially for large tables or tables with lots of columns e.g., alternating white/gray by row or clusters of rows.
Don't repeat text across columns or across rows. Use multi-row and multi-column cells instead.
Keep the table on one page. i.e., make sure cells do not "orphan" to the following page.
Markus Puschel. "Small Guide to Making Nice Tables", Online: https://people.inf.ethz.ch/markusp/teaching/guides/guide-tables.pdf
Stack overflow discussion: https://tex.stackexchange.com/questions/112343/beautiful-table-samples