There are a few things that I've seen people do, which undermines their attempt at promoting their site. This quick and dirty guide will (hopefully!) prevent you from shooting yourself in the foot.

• Don't change the colour of text.
  Changing the colour of your advertising text can make your ad illegible. Some sites have two versions of the same theme, one dark, one light. If you're looking at the dark theme and change the colour of the text to something bright, you're ensuring that your ad can not be read by anyone using the light version. In addition, you simply can not guarantee that the colour you selected will be legible in the future. This is particularly pertinent to roleplay directories and your signature on them.
  
  Do yourself a favour, just leave the text colour alone.
   
• Text size.
  Be mindful of user experience when selecting your text size. Obnoxiously large text in your signature is not eye catching, it's annoying. It's also a good way to ensure that people either turn off signatures when browsing a site, or turn off your signature. You additionally don't want to make your text smaller than the default. First off, if the font size changes it can make the font even smaller and secondly if people can't read it they won't read it.
  
  Doesn't matter how large your text is, if everyone's elected to just hide it.
   
• Image size.
  In megabytes, not pixels. If your image contains several megabytes worth of data, you're ensuring that those with slower internet will not see it because chances are, they're not waiting for it to load. It's awfully nice to have a high quality image, but it's no good if it takes an age to load.
  
  Decrease the quality a little. Condense the image a bit.
   
• Animations.
  Be wary of flashing animations. Just like text size, if your gif is obnoxiously distracting, it'll be blocked or the user's having bright lights inflicted on them will develop a grudge against your site. Be classy about it!
   

That's it! The quick and dirty guide to formatting in your ads.