Installation Tips
1) Use the seat the same way you'd use it in the car (if you use it rear-facing, rear-face on the plane, etc.).
2) Take advantage of pre-boarding. Take that extra time to get the seat installed tightly.
3) Recline the airplane seat back when installing the seat, then bring it upright to get the seat even tighter.
4) For some forward-facing seats, twist the latchplate around once (so that it's "backwards") to make it easier to unbuckle at the end of the trip. Not twisting it may make it nearly impossible to unbuckle the belt. This will also help keep the belt from slipping loose.
5) Put up at least one armrest. This will give the seat more room for installation, and keep it more stable (since resting against an armrest may cause it to tip to one side).
Remember, it's always a good idea to buy your child, no matter what their age, their own seat on the plane so you can use your child safety seat - especially since airplane seats are relatively inexpensive and children tend to be relatively fragile.