Not all attention is positive. Ads with poor creativity can have a negative effect on people's opinions about a brand.
Bigger ads are not always better or necessary. Recommended size: 180 pixels x 150 pixels, a rectangle works best.
Ads that surround content are least effective. People have developed "banner blindness," this is where they chose not to pay attention to these banners.
Good ads are creative and use video (can be expensive) or GIF/JPEG (cheaper). Do not have annoying ads.The least effective ones are simple flash-based ads.
Always use ads that have the option to skip or close.
Try and run multiple ads in multiple positions.
Be relevant to your viewer.
Do not use flying ads or ads that cover content that people want to see/read.
Information from Adage.com