I've had harnesses in a few of my past cars, and I was sad to see that the Yaris doesn't yet have a good harness bar to safely mount a set with. The flat OEM seats and body roll were making cornering tiring having to brace myself all the time :D
A while back I read about the CG Lock (
http://www.cg-lock.com/index.html), which is basically a little clamp that attaches to any OEM lap belt and "locks" it onto you. I picked one up today from Bavarian Autosport in NH (49.99) and have been using it all day. I must say, I'm very impressed.
The install takes a minute or two at most, and it works exactly as advertised. You sit down, buckle up, then cinch the waist belt tight. Before you know it you're pretty well locked into your seat and no longer have to brace as much around turns. With your core locked down I found that I didn't really miss having my chest locked in like a traditional harness (though that has other advantages). And when you're done with it you pull the little lever and it releases. You can also remove it quickly too (though I may just leave it on, it's nice driving with it). It's also SCCA legal I believe.
In all,

for the CG Lock, it's awesome!