When I was producing radio commercials and absolutely HAD to use an outside voice for a part, whenever I got someone who read everything in a monotone (and its actually very common), I would encourage them to read the copy and imagine themselves in a television laundry detergent ad. Those always feature housewives and their families wildly over-emoting about how WONDERFUL and FABULOUS the product is, with WHITER WHITES and BRIGHTER COLORS!
Just inject a jolt of that into your reading and practice doing it for awhile. Of course, once you get the hang of it, you'll want to tune it back to a more realistic level, but its a good exercise. And smile while you read - believe it or not, your voice sounds different when you smile while you're speaking. You may not notice much difference, but (even subconsciously) listeners will.
Other tips for developing the speaking voice: Stand. You won't get as much resonance from the diaphragm if you're bent folded sitting down - cuts the flow of your breath in half.
Bring the sound up from your chest. If you can feel vibrations in your head while you talk, you're 'talking through your nose' - resonating your voice through your sinus cavities which will make it sound flat and tinny. That's the most common cause of 'Ben Stein' monotone (that and lack of inflection - see step one).
Don't mimic professional announcers, whatever you do! They're mostly hams who exaggerate enunciation to the point of 'puking'. Ideally, you want more smoothness than that. The tendency to flat inflection and monotone can work for you if you develop it right - give it a little soft rumble from the diaphragm, keep the volume low and don't forget to emote - you'll get a very throaty, sexy effect. This is true for both males and females.
Rotsa Ruck!