Hey there, Titanium,
I'll second what Some Other Dave wrote - top gear at the lowest speed possible should give the best FE. However you might not want to do your commute at about 45 mph, which is approx the functional bottom speed for 5th in my HX. 40 mph works, but I'm not sure I'd want to do that long term. So, see below for some thoughts re. achieving Lean Burn in the HX. In my car, lean burn is usually available between about 61 to high 70's mph in top gear. Excellent for highway running.
For the HX I really recommend a ScanGauge 2 (or later??) with one of the X-Gauge optional gauges set for a Lean Burn indicator (LB). I might have posted the how-to for that earlier in this thread, but I can send it by private message if you ask. Net mpg increase from LB versus regular air:fuel ratio is somewhere between 10-15% so there's a real gain in knowing when you've gotten it into LB, from the gauge readout.
My car is kinda finicky about going into LB. I've written and talked with others who have HXs, and one guy with a Gen. 1 Insight which also has Lean Burn, and their cars aren't as finicky as mine. My point is, I can't say "put it at x rpm and don't stomp on it, and you'll be in Lean Burn". In my experience, it's not that simple - I wish it were.
You also want an MPGuino to read actual fuel consumption. About $50, but it's truly a do-it-yourself project. Well worth the effort though.
Mods? You get to choose, truly. Have a look at my car's page here:
Details: Lean Burn Civic - 1997 Honda Civic HX Fuel Economy - EcoModder.com
You can see the sequence I did the mods - easier stuff first. I keep putting off some rear wheel skirts. I plan to go down only about half way so I won't need to bow them out. I mentioned it because it should be relatively quick + easy.
And have a look at California98Civic's DX. Not a lean burn car but he's doing very well with it:
Details: Black and Green - 1998 Honda Civic DX Coupe Fuel Economy - EcoModder.com