First, Welcome to GasSavers.
Currently, their aren't any financially viable alternatives to a scan gauge.
On the grill block, most likely you would be fine with a total block in the winter and a block with a 2"*4" opening in the summer. I have an 89 Honda Wagon and I run it with most of the grill blocked. I did notice my electric fan running today, after some highway miles, so I will be opening it up a little.
If you have a temperature gauge, then just keep a close eye on that. If it starts to creep up from it's normal position, open the grill up a little. It doesn't seem to take much of an opening for it to work fine, on my car. I do believe the grill block gives the greatest return for the least amount of effort.
You might want to consider putting your tire pressure higher. At the very least, you should put the max amount the side walls of the tire specify. I've been running about 60 psi in mine, with no problems, for about a year or so, but you should do what you feel comfortable with.
On the gear/speed question, generally, the lower you can keep the rpm, the better the car can do on mileage. However, that presumes you use a very light throttle. At a light throttle, I can go down to about 1400-1500 rpm, in my Honda, but if I need to accelerate, I have to down shift.
Unfortunately, everything about getting better mileage has some relationship to your specific car and how it performs. That is where a scan gauge or something like it would be helpful, I presume. Anyway, awareness and effort are a large part of your options for improvement.