Creaking doesn't necessarily have anything to do with stiffness. I'd lubricate the clamp bolt at the bar clamp on the stem, clean off the bar and stem on the surfaces where they make contact, and even put a tiny bit of oil on the connection (your stem is going to clamp down well on the bars regardless). And is the creaking in your bars or is it sound coming from the headset, the front wheel, cranks, or especially your saddle? Noise travels around and can be a bear to track down.

If you still don't think you have stiffness, try the following:

1. Quill stem? It won't be as stiff if you have a lot of it sticking out of the fork steer tube.
2. Check your stem for stiffness. Often it's the stem that's actually generating a lot of flexibility. If the bars move a bit, remember that they absorb a lot of what otherwise becomes rather tiring or even painful road vibration. Ride a pair of steel Nitto bars on the road once, and you'll see what I mean. Also, try a 31.8 mm handlebar diameter rather than 25.4 or 26.0 -- the bike world is pretty much switching to 31.8 anyway, and it does make for a beefier and stiffer stem-to-bar clamp.
3. If you change your bars, there's a wide range in the aluminum alloys used. If you get bars like Deda 215 or Magic's, they're made with alloys that are very strong and stiff. Old bars like old-style Cinelli's, 3T's from the 80's, etc., are drawn from a very soft aluminum that definitely flexes more. But there's a trade-off. The stiffer alloys will crack if hit hard, while the older bars just bend; if you want to ride away from a fall, the latter is safer. And again, they transmit a lot less road buzz to you. I don't think there's any reason why someone has to ride Nitto steel on the road.