aJon, I've done a few US rentals in relatively recent times. Hawaii and Alaska in June 2010 were the latest. I think the chances of rental agency staff trying to diddle you for a one-way rental charge are pretty remote. I'd just book online with one of the major firms, (UK branch is how I do it, correctly declaring country of residence as Australia). Print the confirmation page and take it with you. If the confirmation clearly shows the total price and the fact that the car is to be dropped off at a different location than where you picked it up, they really have no basis for trying to argue that there is a one-way fee to be added separately.
They are more likely to try and upsell you on insurances - or the 'bring it back empty' option. I think the best approach here is to make it clear you have some idea what you're talking about. I usually put the confirmation email on the counter and say something like "so CDW and SLI are already included in this rate, right?" (which they will be, as I've booked it via the UK branch to get the 'all inclusive' rates). So right off the bat this demonstrates you are not in the most ignorant and gullible group of customers - many, especially foreigners, probably don't know these terms or their acronyms. The confirmation very clearly shows them as being included, so again there is really nowhere for them to go in trying to sell you these insurances which you'll already have.
That may not stop them trying to sell you 'personal effects' insurance or some of the other minor ones that are probably already covered by your travel insurance. Recommend you have a look at the US sites of some of the major firms to familiarize yourself with these terms and what they mean.
You are correct that there is often no one-way rental charge between large city pairs that are relatively close. Same as here - I often do one-way rentals between Canberra and Sydney Airport. Usually (but not always) I can do this without being hit with a one-way fee. Pays to compare rates though, toggling between dropping off at the pickup location and dropping off at the place you intend to drop off. Some firms have been known to not charge a separately stated 'one-way drop fee' but to just apply a higher daily rate across the rental if it is to be dropped to a different location.
Also remember that you can make a reservation now and then cancel it anytime up to 24 (or 48?) hours before you are due to pick the car up. This can be handy to make sure you do not miss out by leaving it too late in busy periods, while still allowing you to take up a better offer if you find one. Just don't forget to cancel the reservation you no longer need!
One more US rental tip - some firms do not charge an extra daily fee for an additional driver (if it is your spouse or life partner) while others do. Some states also ban them doing this. One of the reasons I tend to prefer Avis and Budget (sister companies) is that they don't charge the additional driver fee for my wife. Pretty sure National and Alamo (another corporate pair) do not charge it either. Anyway, as with all these things you need to be comparing apples with apples, not oranges!