The
best way is to find a producer who specializes in that sort of thing. The trouble is that most 'producers' stink at it and charge an arm and a leg to boot.
As a cheaper alternative, try the DIY approach. You have a wonderful resource for ad copy, that being ads from magazines. Find 6-12 products that you think your voice would be good to promote and record yourself reading the ad copy straight from the actual ad. The beauty of this is that you get professional ad copy which is probably going to be better than anything you (or some 'producer') will be able to come up with on your own.
Invest a few bucks in a decent microphone and find some cheap recording software to record it with. To get an idea of what others are doing, try a search on Google for other people's voice over demos.
www.Voice123.com has a ton you can peruse to see what they're all about if you're not already familiar with the format.
With a little work and creativity, you can easily produce a quality v/o demo for less than a couple hundred bucks and you give yourself the ability to constantly refine your work without any additional expense.
Hope this helps and Good Luck!