This is probably the most important thing to know about Rocketry! A stable flight redudes drag allowing a rocket to fly higher.
For a stable flight, the most obvious solution is to add Fins. Large and strong fins are important, but most of all the fins should be as far back as possible. The further back the fins are, the more useful they will be. The reason fins are so effective is because they move the 'cp' of a rocket backward.
The 'cp' of a rocket is its 'Centre of Pressure.' The 'Centre of Pressure' is the centre of an objects lateral area. The Barrowman Equations are a useful way to calculate the 'cp' of a rocket. To stabilize a rocket, the 'cp' should behind the 'cg.'
The 'cg' of a rocket is its 'Centre of Gravity.' The 'Centre of Gravity' is the centre of an objects mass, or its Balancing Point. The 'cg' of a rocket can easily be found by balencing it on your finger. The distance from the 'cp' to the 'cg' is recomended be greater than the rockets width. To move the 'cg' of a rocket forward, extra weight is needed to be put forward. Addind weight not only aids stability by moving the 'cg' higher, but in some cases helps a rocket go higher.
Circular fins move the cp of a rocket backward making a rocket very stable. It's more effective than most types of fins, but it's launcher has to be longer with the release mechanisim lower down.
Stronger and more aerodynamic, the Boat Tail design adds strength because the fin is supported by the Boat Tail as well as the pressure vessel. Boat Tails reduce drag by more efficiently returning the air to its original position since it is more streamlined. Boat Tails are just as important as Nose Cones are in aerodynamics.