Some things that will cause a DSL line to run slower than it should.
If your router is near any electronic equipment, computer case, monitor, TV, etc., try moving it so it's at least 2 feet from any such device. Electronic interference can cause framing errors resulting in packet loss or discos.
Modern surge protectors usually have an in/out port for your phone line. If you run the line to your router through one of these you will be filtering the high end of your DSL signal. Depending on several factors, quality of the surge protector, distance from the Central Office, etc., you could be losing up to 50% of your DSL speed. In almost every case you will lose some speed but in most cases it will be small (most people by cheap surge protectors). Your phone line is protected where it enters the house and you don't need the added protection of a surge protector.
Also once every 4-6 months is a good idea to power down all your equipment, computers and router, for 5 minutes. Power up the router and wait for it to establish a DSL connection with the Centeral Office then power up your computers one at a time. This forces the router to negotiate a new connection with the Central Office so if there have been any change in the field conditions of your service (the power company did some work the next street over or worse there was a telephone repairperson working down the street) this will correct for those changes.
It's also a good idea to run a shielded cable between your telephone interface and the router. Be sure to ground the shield at the interface or the shielded cable is no better than a plain jacketed cable. If you can ground the shield at both ends that's even better.
