Here is a simple tip…
Choose a sensitivity between 3-6 and STICK WITH IT. The way your brain works is like this. It adapts and memorizes your sensitivity. Most pro players play between 3-5 sens and bumper jumper is the most common layout used by pros.
If you always switch your sens, your brain will get confused and this can affect your accuracy when you aim. Just because you have a bad day of gaming, does not mean you should change your sens. Take time to warm up in some customs or an easy playlist before playing against hardcore gamers in Arena or MLG customs.
My general rule when giving lessons about controller settings, goes a little something like this.Sensitivity's 1 and 2 are too slow, do not use them unless you have too! When you play default (3), you have no advantage of being faster than your enemies.
I personally play on 4. I was on 5 when I started to play Reach, then I noticed that it was just a tad too fast for some reason. I was missing a couple shots that I shouldn't have with the snipe and DMR. So moving it down to 4 was good. It is a little bit faster, but still accurate. Anything above 6 is less accurate so I do not recommend it.
In Halo: Reach however, if you like to use the jetpack you will either have to claw on bumper jumper (this is where you put your middle finger on the trigger and your index finger is on the bumper above it. You can use the index to change weapons with Y or use Abilities with X as well. This takes some getting used to but this is how I play if I am not using sprint.
Otherwise, if you like to use the jetpack a lot, you might want to use default or recon as your button layouts. My advice overall is to find your comfort zone and don't change. Try not to blame your gaming on your sens instead go into theater and look where you can improve.
Thank You, Fear your personal Halo Coach.