One of the things you should pay attention to when writing a scientific paper is the layout of the tables. Never, I repeat never, use vertical lines or even a “grid” with lines between all cells. Instead, use only horizontal lines—one at the top, one between the column headings, and one at the bottom. The documentation for the booktab package [pdf], which is appropriately written by a Swiss, explains this nicely. Two more things: the table should be in a smaller font than the text (about 10%) and span the whole width of the text. As scientific papers are always typeset in two-column mode, you may have tables one column or two columns wide. The latter is done like this in LaTeX:

\begin{table*}
\centering\small%
\begin{tabular*}{\textwidth}{@{\extracolsep{\fill}}lll}
\toprule
First column & Temperature (°C) & $D$ (nm)\\
\midrule
Foo & 210 & 10\\
Bar & 300 & 15\\
\bottomrule
\end{tabular*}
\caption{\label{tbl:mylabel} This is the table caption, where you
should explain what identifiers like $D$ above mean.}
\end{table*}

Units go into the header, or into a separate header line (which is IIRC recommended by DIN). The amount of l at the end of the third line is equivalent to the number of columns. That means you should exclusively use left-aligned columns. Also, don’t be afraid to make tables wider than high, or with only one line of data. I have seen this in Wiley-VCH journals, and it comes out alright.

To make the table only one column wide, replace table* by table and \textwidth by \columnwidth in the example above. Do however leave the asterisk in tabular*.