Any ad, banner or not, can be effective or ineffective. Often, it's not the banner itself that makes it effective, it's the placement, the size, the site it's on, and the location where it's placed.
Here are some tips for banner ads:
1. Keep your file size small. Every 3k is 1 second of loading time on slower modems. General rule is to keep your banner ad smaller than 10k.
2. Animated gifs don't necessarily get better click through rates.
3. Use the word "click here" somewhere on the ad... you have to tell the person what you want them to do.
4. Be sure your banner ad is going to appear in a place that's relevant, easy to see, and not too crowded with other ads.
5. Put a special offer in your banner if possible. That will increase click through rates.
6. Find out what the click through rates are for the site you're thinking about putting it on. Industry averages range from .1% to .5%. If you can find a click through rate higher than that, you're lucky. Targeted sites will bring higher click through rates. The banners on my site get a .6% to 1.2% rate.
I probably have more thoughts and ideas but my toddler just woke from her nap. Must go!