Air Superiority means being able to carry out air operations with limited or no interference from the enemy.
First of all, one does not need a higher number of fighters or higher technology than the opponent to acheve air superiority. It sure can be useful, but superiority is not limited to just massing fighter planes and sending them towards enemy air patrols.
Achieving air superiority means denying the enemy air capabilities by whatever means possible.
Set your air to attack ground. Move your planes into the position where your opponent is moving. As the opponent changes direction, anticipate where his planes will end up and give a move order to that position. This will cause your planes to be behind your opponent's planes and absolutely decimate them. Do not give a move order behind your opponent's planes as this will be easily countered. Give the move order directly ahead of their planes.
Introducing some concepts:
The Combat Air Patrol Edit
Many players mass their interceptors or air superiority fighters in a patrol circle around their base. It sure works, but it's largely inefficient, since a good chunk - or possibly ALL planes - may be in a bad position when an enemy air strike comes (that is, presenting their rear ends to enemy cannons).
A CAP consists in a smaller fighter squadron (6-10 planes) patrolling a smaller, triangle shaped area, with a vertex pointing towards a friendly base and possibly right on an Air Refuel Pad. Two or three CAPs can protect from air raids much more efficiently than an 'air armada' formation, simply because planes will never be in a bad position all at the same time, and even when it happens, their smaller patrol range will help them get into position faster.
The CAP can also be used offensively, if layed out toward the enemy base, can help destroy enemy interceptors quickly and efficiently. However, it is very vulnerable to enemy anti-air.
The Cybran T3 Air Superiority Fighters seem especially suited to this mission: being stealthy, they have a much smaller risk of being engaged by enemy anti-air and can catch by surprise opposing planes.
Aggressive Anti-Air Edit
By placing Anti-Air structures in clusters on likely avenues of approach or around the enemy base, the player can effectively seal off airspace and deny its use by the enemy, leaving ground forces to be an easy target for friendly bombers.
Mobile Anti-Air and AA-capable naval units can also be used in patrols, like a ground-based CAP. While this prevents enemy air units from engaging your defense and thus depleting it (specially if the AA is under base shields) this does take away from the flexibility of your defense. Heavier air units such as strategic bombers might be able to conduct at least one bombing run before the mobile AA can engage it, while patrolling planes will be able to engage larger threats away from your outposts and provide air cover in other parts of the battlefield quickly, as needed. If only ground based AA is used, then there is always the chance of the enemy making a hole in your defence with a concentrated attack, and you will be left unable to chase and take down the enemy units. Also, a large part of the threat from the air comes from gunships, which can engage mobile-AA effectively when in sufficient numbers. Thus, a mixture of air and ground anti-air units is often preferable, if somewhat harder to coordinate.
ACD(Aircraft Construction Denial)Edit
Main article: Aircraft Construction Denial
Despite common opinion, air superiority doesn't solely revolve around destroying enemy aircraft the whole time. Oftentimes, oppressing the enemy's ability to construct aircraft (because of unlimited resources and the abundance of engineering power of Supreme Commander, absolute denial of construction points is impossible) can seriously hinder the opponent's ability to execute most missions including air-to-ground support.
This is most easily done by`attacking the enemy's structures associated with airborne missions. While Air Factories are -obviously- the target with the best strategic value for ACD, those building are usually near the center of the base's unit construction zones, and thus heavily shielded and defended, up to the point that the size of the force needed to destroy a large percentage of the enemy's factories may be just enough to actually finish the job directly and assasinate the enemy ACU.
However, the possible targets for ACD can be many more. Especially on the larger maps, where a succesful air strike may require the use of multiple checkpoints, the destruction of scattered Air Staging Facilities, Cruisers, and Aircraft Carriers may delay the advance of air forces.