The 4 things that make a Prius good at hypermiling are:
- Staying in the "atkinson" bit of the valve timing, you'll probably need to go slower though, but I don't know. Perhaps get a ScanGauge to find out? I'm pretty sure thats why they put an 1.8 in the later models, to get the Atkinson thing going even at higher speeds.
- Pulse&Glide, the hybrid system makes this extremely easy to do, no need to operate a clutch, turn off the engine manually, etc.. If you drive in places with hills, its much easier to do this though.
-plug-in pack, replaces fuel with electricity, and keeps the engine in atkinson mode even at higher speeds, so better bang for your fuel.
the battery cost is hard to justify though, unless you live in a place with crazy gas prices, put hypermiling scores above cost/mile out of principal, or get a really good deal on your batteries. (Perhaps assemble a pack of used cells or something.)
-stock aero is already pretty good. (Theres always improvements, but still, its not bad to start with.