This instructable shows a simple way to raise and lower a plunge router inside a router table.
I found it difficult to set the height in my router table due to the resistance of the spring. When released, the plunge router spring would extend to its maximum, removing any kind of adjustment that had been previously made.
This instructable uses simple materials from a hardware store, and some fairly basic hand/power tools.
NOTE: I have my router table screwed to a piece of wood. The height adjuster may lift your whole router table instead of just the router if you don't have the table screwed to a piece of wood. I haven't tried it with the table not attached to a piece of wood.
NOTE: This design may block the vents of your router, or may interfere with speed knobs, etc that are on the bottom of your router. If you use this design, be sure that you are not going to damage your router by putting pressure on its base, or blocking its air vents. For prolonged periods of running the router, it would be a good idea to remove the height adjuster. Additional holes in the pipe flange may also provide adequate ventilation.