There are "standard" sizes for banners, and you need to make sure you adhere to those standards before creating them. The most popular size by far is 468X60. Some of others are:

234X60 Half Banner 120X240 Vertical Banner

Popular button sizes are: 120X60 and 125X125 and a micro size of 88X31

You'll want to make a few in a variety of sizes so you already have right size for job at hand.

So what do you need to include on your banner? There are a variety of things you can do when designing to ensure clickability.

1) Use actual words "click here" somewhere on banner as a call to action.

2) Use some form of animation- Movement catches eye

3) Make sure your banner loads quickly by keeping size down to no more than 10K if possible.

4) Offer Something Free

5) Tell them just enough to pique their curiosity so they'll want to click and learn more.

6) Studies have shown these colors work best Orange, Green, Red, Blue, Black and Purple.

7) Create a sense of urgency by stating something like "Last Chance," or "Limited Time Offer."

-- Ad Copy Tip #1:

Your headline is very crucial. Yes, headlines can even affect response to your web pages. Many copywriters spend more time on headlines than they do on all other elements of an ad combined.

The purpose of a headline is to catch your targeted prospect's eye and motivate them to continue reading rest of ad (or other type of marketing communication). If headline doesn't do its job, ad will most likely fail.

-- Ad Copy Tip #2:

Instead of trying to appeal to anyone and everyone with your headlines, they should be targeted toward prime interests of your best prospects. There is little point in drawing attention of those who have no interest in your offer.

-- Ad Copy Tip #3:

Since people tend to delay action, ad response can be improved by making special limited-time offers that encourage them to act NOW. (For sake of honesty and credibility, use a real deadline.)

-- Ad Copy Tip #4:

Your ads should include a clear "call to action," which encourages your prospects to take ACTION toward buying your product or service. Yes, make it unmistakably clear exactly what next step prospects should take in order to benefit from your product/service: place an order, request more information, or whatever next step in sales process may be.

-- Ad Copy Tip #5:

For maximum response you should constantly test and refine your ads in order to determine which approaches work best. Once you have a proven "control" ad, you should continue testing in order to increase response even further.