Today, most purchases start with a Google search. Long gone are the glory days of the phonebook; customers now rely on computers and smartphones to find local companies in their area. Fueled by interactive maps, reviews, photos and local business listings, online powerhouses like Google and Facebook have the power to turn prospects into loyal customers.

That's why Google's paid search service, Google AdWords, is one of the best weapons in capturing local leads. Yet, as my company recently discovered, many small business owners aren't harnessing the full power of AdWords. After surveying over 600 small business owners, we found that 40 percent of respondents don't feel their use of AdWords yielded any new leads while 33 percent said AdWords generated very few leads.

The one common thread between small business owners whose paid search campaigns have failed? They all report weak online presence--which is crucial to the success of an AdWords campaign.

With these findings in mind, it's critical to explore three of the most common mistakes small business owners make when launching AdWords campaigns:

1. Failing to optimize their website for mobile devices.

For search purposes, simply having a business website is not enough--your website must be optimized for mobile devices as well. After all, the average consumer spends more time on their phone than on a computer. An AdWords campaign that includes smartphones and tablets is a great way to get your business in front of these mobile customers.

Before investing any money in an AdWords campaign, however, business owners must make sure their websites are ready for mobile visitors. After all, if your ads direct smartphone users to your website, you want them to have a good experience and a positive impression of your business when they get there. But according to the same Manta poll, 62 percent of small business owners admit they do not have a mobile-optimized website.

Google actually penalizes websites that are not mobile-ready in search results, which should give small business owners all the incentive they need to upgrade their site functionality.

2. Not building their reputation with customer reviews.

Customer reviews are key to attracting customers online and establishing your reputation as a trusted local business. Yet, just eight percent of AdWords users said they had customer reviews on Google; and a separate poll we conducted found that 57 percent of small business owners do not actively encourage customers to leave reviews.

Though an AdWords campaign may drive increased brand awareness without customer reviews, it's unlikely that prospects will convert if your business doesn't come highly recommended. Not only can reviews entice (or turn away) prospective customers, they also provide Google with a ranking signal--meaning that businesses with more reviews often rank higher in organic search results.

When it comes to online reviews, small business owners should prioritize quality over quantity. To do so, don't be afraid to solicit reviews from satisfied customers by simply sending follow-up emails asking them to share their positive experience with your company.

Keep in mind that while this strategy may generate some favorable customer reviews, most businesses encounter the occasional bit of negative feedback online. In the event that you do receive a dissatisfied review, it's essential to respond to each customer individually and address their concerns.

Owners should provide both a public and private response that maintains a courteous tone and acknowledges the customer's dissatisfaction while offering a solution (such as a discount on the next purchase, free shipping, or a refund).

3. Falling outside of Google's Local Three Pack.

For small businesses looking to maximize their presence in search engines and execute winning AdWords campaigns, securing a place in the Google Local Three Pack is a must. In other words, every local business should work towards becoming one of the three businesses that appear at the top of the page when a customer searches for a business in a specific location (e.g. plumbers in Kansas City).

While making this coveted list may feel like an unattainable goal, there's a prescribed formula that businesses can apply to reserve their spot. (For a step-by-step guide on earning a place in Google's Local Three Pack, check out my previous column on the topic.)

If you've committed any of the mistakes outlined above, you're not alone. To avoid falling victim to failed paid search initiatives in the future, make sure to complete the necessary preparation before planning your next AdWords campaign.

By building a mobile-optimized website, seeking customer reviews and obtaining a spot in the Google Three Pack, you can truly maximize your AdWords investment while bolstering their business' search visibility.