When you add a promo bar that is uniform through the site though, here are a few things you should keep in mind to take full advantage of what it can do for the objective you have in mind.
Of course, you have to have the usual content and design guidelines in place - like a catchy text, an appropriate size and placement (top and/or side) for the bar and so on. Here are a few non-intuitive suggestions that might work towards making that promo bar more effective.
This is a maxim that cannot be over-emphasized. How often have you seen sites with so many promotions trying to vie for your attention that you are confused and/or lose interest and end up leaving the site sooner rather than later?
Try using only one promo at a time to make it more effective.
Also, reduce the text that shows up on the bar itself to the bare minimum.
Keep it not intrusive.
You don't want to make your promotion at the cost of the visitors' site experience. The trick is to spot that perfect position on your site that is prominent yet it is not in the face of a visitor who has already seen the promotion.
In fact, to help gain the most out of it, merge the design of the promo bar with that of the page. Extensions like JTag PromoBar allow a lot of customization in terms of color and fonts to enable you to do exactly this.
Remove navigation bar.
The site's navigation bar is most likely competing for your visitors' attention. The good news is that you have complete control over this competition. Experiment and get rid of the navigation bar for the duration of the promotion. This goes hand-in-hand with the two tips mentioned above. It decreases clutter on the site and keep the promotion non-intrusive.
Have a call to action button.
If you are looking for an action from the visitor, prompt them into it by highlighting the call-to-action button. Whether it is subscription to your newsletter or your social media pages; whether it is to buy a certain product or to drive traffic to a certain page – put it in the promo bar.
Highlight a non-intuitive side-effect.
You might have a new product that you are promoting in the said bar. But, more often than not, the visitor gains benefits more than the obvious. For example, free shipping for the new product or say a coupon for future purchases along with purchase of the promoted product and so on.
While a site-wide promo bar might not seem like an effective way to carry out a promotion activity. It can be useful as a part of your overall promotional efforts. In fact, that you cannot do much should be taken advantage of. “Less is more” is of more importance here than ever.