Interference drag is the result of an interruption of the boundary layer. The abrupt disturbance causes much drag. Such causes of interference drag are as follows: a protrusion, cavity/gap, joint, bump, or wavy surface. The most prominent source of interference drag is from the fins. Obviously, a rocket with triform (3 fins) fin configuration will have 25% less interference drag than a rocket with cruciform (4 fins) configuration. Also, interference drag will be reduced with a fillet in the fin root connection to make the interruption less sharp.
Also, the air has to close behind the speeding model with as little disruption as possible. If the rocket is going faster than the air can close behind it there will be a negative pressure drag due to the partial vacuum tugging at the back side of the rocket. So sometimes a boat tail or transition to narrower diameter will reduce these effects.
The drag due to lift can be expressed with the following diagram:
The induced drag becomes greater if:
-The fins are swept back
-The fin span is shorter
-The fins have a rounded tip (leave the very tip square, but give the leading and trailing edges a good airfoil)
Summing It All Up
When studying drag, one realizes that there is no "correct" or "ideal" way to make a rocket. When you include a feature that is supposed to help in one area in aerodynamics, you're going to have to make a compromise in another area. But with insight as to what causes drag, you should be able to come up with a happy medium.