When you consider the percentage of shoppers who will visit your website from their mobile devices is expected to increase to 15% this month, you may want to expedite that usability testing project and optimize your website for mobile devices. In October, nearly 10% of all online shoppers made their purchases using iPhones, Droids and similar devices, up from 3.4% during the same time last year, says John Squire, chief strategy officer of IBM Coremetrics.

Despite this exponential growth, few digital marketers test their websites on mobile devices. According to Econsultancy’s 2011 Conversion Rate Optimization Report released last month, 28% of those surveyed tested their websites on mobile phones while only 16% checked how their websites rendered and behaved on tablets like the iPad.

If forecasts and statistics don’t grab your attention, then Google’s new GoMo initiative should at least get you to consider your mobile strategy right away. Google has tons of data on how much traffic is coming to websites that use its own Analytics program and likely sees this dramatic trend as well. Any decent web analytics program will show you how much traffic your website currently gets from mobile devices:

While many companies will get caught up trying to get listed in the App store, you can do some simple things to assure your shoppers have the best possible mobile website experience—helping to improve your conversion rate. Start by looking at your own analytics, and then run some basic, straight-forward tests. Here are three ideas:

1) Add a store locator or special discount for mobile website visitors who likely are doing in-store research on where they can get the best deal.

2) Use larger navigation or a mobile-specific style sheet that takes advantage of a smaller screen.

3) Target an exclusive “Droid Only” or “iPhone Only” offer that could turn your mobile browser into a mobile buyer.