You've received some good replies. I'd also suggest you consider going to a commercial casualty CIC or other class. It gives you a good overview of several commercial lines classes of business and you'll have the ability to network with many other agents and agency professionals. If you like it you should seriously consider getting your designation.
It's hard to be specific as to advice without knowing where you are located geographically and the size and type of agency you work for currently. I'd suggest that if you're an employee you check to see if your employer will pay for some or all of your training. I'm assuming you feel like or know they won't, in which case you should also talk with other local agencies to see what your options are in that regard. Most good employers are excited to think that an employee is eager to learn more and grow in this business, so take advantage of that if you can.
Finally, if you're really serious about learning new coverages, commercial or otherwise, there is a ton of info on the internet in addition to other courses you can take as self study. Look at the IRMI website for example http://www.irmi.com/
, or http://www.scic.com/
. Or better yet, check out your local or state agents association and see what sort of education they offer.
You can probably make a good living as a personal lines agent, but I would have to think you can earn more and it would be alot more interesting if you decide to move over to commercial lines. No matter what you do, you're going to have to study a great deal to be the best agent for your customer - there's no easy way to get there. Good luck.