There is no other aspect of R/C car racing so full of strange, unwarranted practices than breaking in a R/C car electric motor. From Victor's motor break-in program on their Hi-IQ series of chargers to bench run-in to mysterious break-in fluids, motor break-in is rivaled only by Ni-Cad care in opinions, beliefs and "Magic," all justified without any attention paid to facts.
First, ask yourself "What is there to break in on an electric motor?" There are 4 points of mechanical contact which move on a motor -- the two bearings (running against the shaft) and the two brushes (running against the commutator). Audiophiles will tell you that speaker wire which has been "broken in" will sound better than fresh wire, but there is no scientific evidence which indicates that there is a measureable difference in the properties of copper after it has conducted electricity than before. I will go on the assumption that copper behaves the same after conducting electricity as before. The single most expensive component of any motor is the armature, so the whole point of breaking in a motor, I would presume, would be to preserve the life of the armature as long as possible.
If the motor is equipped with ball type bearings, then there will be no wear on the armature shaft with normal use. However, if the motor is equipped with bushing type bearings, then the armature shaft will wear with use, although at a substantially lower rate than the bushings. In this case, concievably there could be a need to wear in the bushings so that the armature spins freely within the bushings.
Indeed, breaking in bushings is something that should be done on some bushing equipped motors. Wether or not bushings should be worn in prior to use is determined by spinning the motor with the brushes removed. The friction present is a good indicator of how tight the bushings are. Testing armature end-play and feeling the fiction is another acceptable method. In rebuildable bushing equipped motors, the armature should be removed and the bushings bored out using a slightly oversized reamer. If you use a drill bit to drill out the bushing, the motor will be sub-optimal since drill bits tend to drill holes that are roughly triangular, whereas reamers create round holes. You should be able to obtain a 1/8" oversized reamer from a good industrial tool source (located in your yellow pages). For non-rebuildable motors, the bushing break-in is accomplished by putting valve grinding compound (available in massive quantities for next to nothing at automotive parts stores) into the bushings and spinning the motor. This will wear both the armature shaft and the bushings, freeing up a tight fit. The compound should be applied using a thinning carrier (like motor cleaner) to allow the compound to penetrate into the fit where it will do its job. The process will start out slow (because there is not much room for the compound to get in), and will rappidly accelerate as the tolerance decreases. Make sure to check the fit frequently. Compound in the bushing may make it appear as though the bushing and shaft have not worn as much as they really have, so you may want to thuroughly clean the bushing to confirm the amount of play. The motor should be turned by a slave motor throughout this process with the brushes removed to preserve the finish of the commutator. Low armature speeds will also prevent the compound from flinging out of the bushing. Care should be used to prevent the bushing and shaft from wearing to the extent that the armature stack rubs against the magnets. Noticable slop tends to produce good results in 1/12 scale motors run with 4 cells at low speeds in clean environments and very thick oil; less slop seems to work better in higher power applications. Make sure all the griding compound is removed from the bushings prior to putting the motor into service, or very short bushing life will be the result.
The only other part of a motor to break in is the brush/commutator interface. As the commutator is not replacable as a seperate component, its life should be preserved as best as possible. Therefore, it is desireable to make the brushes wear to the shape of the commutator with minimal wear on the commutator. If you were to put the brushes in a motor and spin the armature of the motor with a slave motor, this would be the ideal preached by some. This has no electrical power flowing through the motor you are breaking in, and the only action to break in the brushes is the wear between the commutator and the brushes. This will take a very long period of time, and will cause extensive wear to the commutator before the brushes seat. The high spots on the brushes will have lots of time to wear grooves into the commutator. Another method preached would be to run the motor with no load on it with low power (4 cells usually). This will have a marginally lower break-in time than the no-power break-in method. This procedure takes a great deal of time again, and will also cause a great deal of commutator wear just waiting for the brushes to seat. The best way to break in and seat the brushes in a motor is to just put it in the car and run it. The load of the car on the motor causes high current to flow through the brushes. In a non-seated brush, the portion making contact will heat up to a high temperature, and will quickly wear away, exposing fresh brush material behind it. This has the result of a minimal time for high spots on the brushes to wear the commutator, and causes the brushes to seat very quickly. Once the brush has seated, its wear returns to normal with minimal commutator damage.
Another method for breaking in motors is the dipping method. This involves dipping a motor in liquid, usually water, in a jar. It's often connected to 4 or 6 cells when dunked. The fluid around the armature causes a high load to be placed on the motor, and this can easily draw upwards of 30 amperes. This puts a large load on the motor, again causing the brushes to seat quickly. However, water can get between the commutator and brushes, and may cause some arcing to take place; firm brush springs will minimize this. Arcing is what causes the most commutator wear, so it is best to minimize arcing. Some people will blindly state that "water and electronics don't mix; anyone knows that", but these people don't understand what is going on. Pure water is an insulator. Although we are not dealing with pure water, the water we would be using (fresh water) has a very high resistance. Much higher than what would short out a motor run at 7 or so volts. The major problem with water is corrosion; if you ever water-dip your motor, make sure it is thuroughly dried before being stored. It can be dried by running the motor thus causing it to heat up, but since the armature stack is iron, it will rust. Water dipping will also thuroughly clean all the crud out of your motor, but it will not clean the commutator..
Comm Drops are also hearalded as some sort of magical break-in fluid. Comm drops are a lubricant that is added to the brushes which lubricates the brush/commutator interface. Since this interface is subject to very high temperatures, the oil will burn off and leave a dirty surface behind. Not only that, but the comm drops will tend to cause brush dust to clump and collect in the commutator slots. This clogging can cause the commutator to short, which is a bad thing. When using comm drops, make sure to regularly clean the crud out of the commutator slots using an appropriate tool, being careful not to scratch the commutator. Comm drops will also tend to soften the brushes somewhat, causing them to wear faster and thus seat quicker. However, the reduced friction negates this somewhat due to reduced mechanical wear, so wear is primarialy increased when a load is applied. Comm drops tend to have a positive short-term effect, but tend to be detrimental to motor performance after wearing off.