You can probably do a lot of the maintenance yourself. Jeeps are easy peasy! My mpg increased when I put 35's on (I was running 33's with 4.10 gears previously. The 35's put my rpms down a little bit, helping economy)... but it all depends on how you start your build. I personally don't think CAI and the like is worth it.
The best thing you can do for fuel economy in a Jeep is change your driving habits! Let your jeep slow down going up hills... coast down hills... don't accelerate fast, and take your time going from A to B. Keep up on your maintenance, tire pressure, rotation, etc etc.
As far as a lift goes, there are so many different approaches. You did say trails, so I would look into maybe a 2.5" lift if you want to make your Jeep look a little bigger with a little better performance, without getting into the 4"+ lifts which generally require a lot more work.