The "WD" in WD40 stands for "water displacement". It was originally created for marine applications where water caused corrosion and oxidation. The main component is kerosene, and you are right David, WD40 WILL initially solve the problem but because it evaporates it turns into a sticky sludge that will make your problem worse. Better products are silicone and teflon sprays which have a medium which evaporates leaving the teflon or silicone as a coating. My favorite in the US is "TRI-FLOW" which comes in a black can. Many bicycle shops a equipment refitters use it.

Now, doors. In my particular field I deal with metal and aluminum doors in commercial applications where grinding tops and bottoms is not feasible or too labor intensive. Here's what I do:
For doors which are rubbing on the top at the header you can remove one leaf of the top hinge and and use cardboard to ship the hinge out. This will lower the top of the door but it will also cause the leading edge of the door to move closer to the jamb. If the door fits tightly to begin with then this method will only create another problem.
For doors which are dragging on the threshold you can use the same shimming method on the bottom hinge.

For doors that don't seem to close all the way but spring open if not latched, I take a screwdriver and, while standing on the side of the door where the hinge knuckles are visible, I put the shaft of the screwdriver in between the two leaves of one of the hinges and close the door till it gives some resistance. This will slightly bend the hinge and cause the door to cant further into the door frame. You may have to do this to all three hinges before it works, and I would suggest extreme caution when doing this to hinges smaller than 4" by 4" (meaning MOST residential hinges) as it may just pull the screws out of the wood.