…I personally dropped my yearly usage from 900 to 750 by adding attic insulation and installing a new high-efficiency furnace. {still more than I’d like though}

Other passive options:
Insulate & seal leaks in ductwork that goes through garages / attics.
Let the sun in during the day. Close your window treatments at night.
Plant shrubs in the yard in the right spots to serve as wind-breaks.
Turn down your hot water setting in the summer. I dropped summer gas usage by about 25% by setting this down, and we didn’t miss the hotter water at all in the summer.

Bigger options:
Solar hot water heater to supplement traditional means.

Geothermal heating with a heat pump. {This is the biggest way to drop natural gas usage since you’ll be getting all your heat from electricity. but who wants to take on $30,000 in loans to build it? if you do – its has a fairly good return on investment. plus interest rates are rock-bottom right now}

Here are some other options I’ve used that weren’t mentioned in your post:

What help me find out what worked was taking a frequent measurement of gas usage. {daily in winter, weekly or monthly in summer}

Its too easy to be out-of-sight out-of-mind when the only time you get feedback is when your bill arrives. Daily readings by hand probably isn’t for most people, but if you spend a few minutes each week to take a reading, you’ll know what’s working and what isn’t. Daily readings really showed me last year that the thermostat is the most important place to control your usage.

Some thermostat suggestions:
1) Play around with your overnight temperature until you find the lowest temperature that suits everyone in the household. Don’t be afraid to go too far for one night. You can always adjust mid-night, or the next day. Every degree is about a 3% savings. If you find out you can be happy in 5 degrees cooler than you thought you could – you’ll lower use by 15% without a big inconvenience to yourself. That translates to big savings over the season to find the good setting. We kept ours at 60 degrees, with a 65 warm-up for when we had to get ready for work. 60 was plenty last year… but with a new kid this year, we’ll be going with a higher setting.

2) Hold your thermostat at a low setting if you’re going away for the weekend. This is especially effective if you travel a lot and live alone. Most furnaces can make a house comfortable within a half hour or hour. Its a big waste to keep the house warm when you’re not there. (Last year, I found that keeping the house at 60 one weekend while I was away used about 1/2 as much as when I run my normal program during similar weather).