My wife drives our '99 TL -- as you know we lucked out with getting the 4-speed automatic and its reliability.
We have about 130K miles on it, and the Catalytic Converter needs replaced, so all of my experiments have yielded zero improvement (except tire over-inflation).
* Of course, start there, pump up the tires.
* Next, if you don't mind the sound at VTEC, take out the air silencer -- it's extra weight and frankly is just an inhibiting muffler for the intake.
* Clean the carbon out -- use a product like "Sea Foam" or AutoRx. I've used Sea Foam on the TL -- it's basically dumped into the vacuum brake booster line into the intake (Caution: it creates lots of white smoke, but that's just the carbon burning off and the chemical). Just follow the instructions for intake cleaning. Then go out and run 'er pretty good. It'll look like you threw a rod with all the smoke, but that goes away after a few miles.
* After the Intake is clean, install a PCV catch device to collect the gunk that probably created the problem in the first place. A good DIY: http://www.gassavers.org/article/pcv..._long_run.html If you need to find the PCV valve, let me know. I can provide pictures (several covers and parts need to be removed to get in there -- I would recommend replacing the PCV line with a longer one instead of splicing and inserting the device, because it's a tight corner, and could get crimped). I've used this on our Integra, and it has collected several ounces of gunk.
* I subsribe to the notion that higher Intake Air Temps yield better economy. The beauty of the TL is that it has a little heater under the air filter. Basically, there's a trap door that's probably opened at cold starts to get warm air in the intake to reduce emissions (it has coolant line that runs to a little radiator-like device). The trap-door can simply be forced open by using a long screw through the door into the plastic cover to force air to come through the radiator all the time -- then you've got an easy HAI. Make sure you use Premium Fuel here, or else you may get pinging, especially at VTEC or at the VTEC transition on RPM decrease.
* Otherwise, bolder mods could be to lower drag, some claim synthetic fluid (transmission, oil, PS fluid, etc.) reduce friction, replace the 02 sensors (depending on mileage), or spark plug mods.
* Lastly, driving style. I always use the auto-stick to keep the RPMs low (but I just got a Scan Guage to verify my theory on that) -- make sure the load on the tranny is low at shift, so time the shift and lift the throttle slightly so the tranny doesn't get toasted (i.e. don't floor it at low RPM and keep shifting it into 4th when it will let you, or else I can only imagine the torque it's dealing with)
Otherwise, I haven't messed with our TL a whole lot, but let us know if anything works out.