PATIENCE HONDA BOY!
4. Remove the seats, A/C compressor if you don't need it along with anything that's dead weight. If you're 300lbs, lose 50lbs and you'll thank me in twenty years while you save $10/yr in fuel from the additional weight.
5. Amsoil (I recommend), Mobil1 are the industry leaders in synthetic oil. You don't need anything to convert from conventional oil to synthetic. If you don't add oil between oil changes switch to synthetic. Some people get an engine flush done if they have super high mileage, but it's not necessary. If you have the means, remove the oil pan and clean it out thoroughly along with the oil pickup screen.
6. I wouldn't worry about flushing transmission fluid at 140K. It's only bad to flush high mileage Dodge transmissions, as they're prone to fail shortly after. Regular trans fluid changes every 30K or two years will give you maximum life out of the trans. There are even synthetic options, such as Amsoil ATF, Royal Purple and Mobil1 has a version out now too. Again, I'd recommend Amsoil.
7. Adding power can be done in many ways, the most have already been covered; intake & exhaust. You can also add a performance chip if you can find one available. I do not recommend JET chips, they're over priced and at best will only give you 5hp for your money. Running synthetic axle fluid and transfer case fluid along with synthetic engine oil will give you best results for the least amount of $. Keep tires properly inflated and rotate/balance on a regular basis for best mileage.
As far as lowering weight goes you can find good, inexpensive aftermarket alloy wheels (15/16" whatever size you take) that can aid fuel economy. In some cases, stock alloy wheels can even weigh more than steel equivalent! If you don't need the rear seats, unbolt them and store them elsewhere. The average seat (split bench) runs close to 80lbs. If you have a decent budget, an alloy hood can also help, but will run $500-800 with paint and save 10-20lbs. If you want to get drastic and aren't concerned about road noise you can remove the rear interior panels, along with the spare tire as you mentioned, the seats and anything else that is held in place with screws or plastic tabs.
When I took my Durango to the track I weighted it stock with myself, and after I stripped it down and I saved myself nearly 400lbs. Made a difference! Putting it all back in so the wife and kids had a place to sit took some time, however.
Oh, you can also check out Performance Parts Online for Cars and Trucks - Performance Center
or Summit Racing - High Performance Car and Truck Parts | 800-230-3030
for other performance add/ons etc.