If you're planning on doing a higher elevation hike than you usually do (perhaps in the 7,000 - 10,500 range, not much higher) there are some simple things you can do to make sure you don't get altitude sickness, and to make the hike a bit easier.
These are just my tips: if you have anymore, please add in the comments so we can all learn from them!
1) Increase your hike frequency and distance little by little. While this won't help with elevation, it will help with your general well being which is never a bad thing!
2) Take some hikes are gradually higher elevations (like 4,000 or 6,000 if you can get close enough!). Little bits help. You'll at least then know how much your body reacts to some elevation changes.
3) If you focus on one thing it should be carido. Wear your pack with some weight and climb hills or stairs. Running and biking are good, too, but they do not work the same muscles.
4) Drinks TONS of water before, during, and after the trail. You'll thank yourself for this.
5) Get yourself to a swimming pool: nothing gets yourself good lung capacity or cardio like swimming does!
6) Sleep the night before the hike begins at the highest elevation possible to acclimate.
7) Learn good techniques for climbing at altitude: gradually reduce your pace as you gain elevation. Immediately after noticing breathing difficulty, slow your pace down.
8) Ideally, spend 2-5 days acclimating, and don't sleep at changes in altitude over 1,5000 ft between days to help your body adjust.