I found (in my past jobs working for truckload carriers) that ads didn't work nearly as well as word of mouth. We told our current drivers that we were looking for more drivers and gave them an incentive.. $250.00 for a good lead and another $250.00 if the lead panned out and they were hired. After that they got another $250.00 if the new hire remained with the carrier for at least three months. Some drivers did really well with our incentive program.. one of them got us 20 good drivers in the span of a year and boosted his earnings by 15K for that year.
About ads, we used Truck News and Over the Road. Results were disappointing; however we found that results were better when more information was provided. But we eventually stopped using ads as the cost was prohibitive compared to the return we got. Maybe also look at advertising in your local newspaper.. we did that and found it to be a lot cheaper and somewhat more cost effective than the trade publications.
Hands down though, a referral program is most effective.. drivers listen more to other drivers than they do to recruiters or what's in an ad.