As far as I know, you can't get a stronger battery without changing the shape of the back cover. Other wise HTC would have put a bigger battery in there.

 

Buy Mugen Power 1550mAh Extended Replacement Battery For HTC Hero (GSM) [HLI-HEROSL]

Mugen do make a 1550mAh battery, which would give you a bit of an increase. Not sure whether it'd really be worth it, I can't see that extending overall life by more than about half a day.

 

You can always get a Power Monkey:

https://powertraveller.com/iwantsome/primatepower/powermonkey/

It's cheaper than most good batteries, small, convenient and packs a good punch (with a maximum output current of 1.2 A running at a consumption of 2200 mAh, it's like having almost three new Hero batteries always available).
And you can use it with other stuff as well (iPods, PSP's, etc.).

 

I'm a wee bit confused. Wouldn't 2200 mAh be less than two standard Hero batteries, with them being 1350 mAh and all?

 

You're right on that one, it was a typo. I meant to say "almost two new Hero batteries available". Sorry about that.

And the Power Monkey should deliver at least that. If you look at the specs, it's rated with a capacity of 2200 mAh for a maximum output current of 1.2 A and a running voltage between 4,5 V and 5,5 V.
The Hero battery is rated at 1350 mAh with a constant DC voltage of 3,7 V (at least it's what's stated on the back of the battery).

So the Power Monkey should roughly be able to charge the Hero twice. That's probably enough for my usual three-to-four-day trips around the country. But I'm still waiting for my Power Monkey to arrive. I will share my first-hand experience once I put my hands on one. For now, I can't think of something better.

 

Actually, because the Monkey runs at a higher voltage (which it has to in order to be able to charge the Hero), it will run through that 2200 mAh faster, but due to inefficiencies of the charging circuitry, there's a substantial bit of loss.

I'd say the Monkey with its 2200 mAh will be able to charge the Hero with around 1800 mAh, so close to a full charge and a half.