I don't know the
real reasons behind your choice of Yaris.
If it's truly fuel economy, I suggest an alternative: Scion xA. If you don't mind slightly used (save on first-year depreciation, dealer fees, and possibly sales tax), Toyota Echo is another choice. Yaris, xA, and Echo have similar, if not the same, base engine.
Don't mention you're paying cash. Usually it's difficult to negotiate for new vehicle like this, especially since it's almost as fuel efficient as hybrid. You may want to check some credit unions in your area to see if they have special fleet pricing on it.
If you want to be brave, go to Toyota dealer, pretend you're servicing your vehicle, take their courtesy shuttle to work or home, wherever. In the middle of chit-chat with driver (usually a retiree here in AZ), ask him/her if they get special pricing for Yaris, and if he/she can help you getting it.
And, don't buy Toyota extended warranty from your dealer, before you compare with this:
http://www.spoofee.com/forums/showthread.php?t=18504