Soldier is all about managing choke points.
The offensive classes do have a lot of speed, the key is to minimise the effect of that speed by good positioning and superior awareness. Don't ever put yourself in a position where you have to shoot 'after' someone, always try and get them coming towards you or next to you. If your team has a layered defence or you at least have other players around to mop up if players get past you then hold your position and make sure no one gets past you without at least getting quite badly hurt.
Don't be afraid to use the Shotgun either, one of the key elements of being a good Soldier is the ability to switch weapons on the fly and use their strengths when the need arises. The Rocket Launcher does good damage but you only get four shots and rarely get time to reload before a Medic or Scout is long gone. Use the splash damage of the Rockets to manipulate the movements of those that you shoot. Once you get good you will find yourself shooting a Scout up in the air and dropping another Rocket exactly as he lands, finishing him off. Alternatively you can start with a Rocket shot and switch to the Shotgun to finish since very often a Scout or Medic can ride your Rocket past you.
There are many other things I could say but I can't give away all my secrets
If you find the enemy Medic/Scout has to waste a conc on you then this is quite a good indicator of how well you're doing as a defensive Soldier since those classes rarely give up their concs in such a manner in FF, certainly not like they used to in TFC.
There are maps where it's quite hard to be a Soldier, Aardvark probably being the most obvious of those. On a map like this you have to position yourself well in order to make your job as easy as possible which in turn should make you as effective as you can be.