Make it readable. Of course the grammar will be great, but imagine someone having only a small amount of time to read through your summary, you need to ensure that they can read your summary effortlessly.
Make it relevant. Have up-to-date and relevant positions, experiences, volunteer work, and extra curricular activities. The summary is going to sum your current self up quickly, so leave the old stuff behind.
Be concise. No matter what format you choose to use (some formats to follow), always be concise. This does not mean writing a short summary or a bulleted summary, but it does mean to be direct about what you strive for, are good at, and who you are.
Those three items should keep the summary looking good for anyone to read. In saying that, there are a few different formats that you can go for if you are not sure where you would like to start. One is short blurbs and the other is a paragraph format.