Most of the answers above explain how to keep a tip tinned and shiny. I went back and read your orignal message again - you are complaining that metal is being eaten away from the tip and you are losing the point on the tip.
An eroding tip is frequently caused by the operator melting the solder on the tip of the iron when soldering a joint. The solder and flux react with the metal of the soldering tip, and a small amount of alloyed tip material is transferred into each joint you solder.
The proper way to solder a joint is to heat the JOINT using the iron until the JOINT is hot enough to melt the solder. Flow the solder on the joint itself - not from the tip of the soldering pencil to the joint.
Condition your new tip by tinning it. Then, as you use the tip, clean it frequently by wiping away the excess flux, dirt, and metal oxides that accumulate on the tip. The easiest way to wipe the tip is to keep a small damp sponge near your work area, and simply wipe the tip across the sponge periodically. Re-tin the tip if it stops looking clean when you wipe it.
Obviously, better tips are made of better materials. A cheap plain copper tip is not going to last as long as one made of a hardened, plated material. However, if you follow the proper soldering and cleaning method above, you can extend the life of your equipment.