One fear that most model railroaders have in starting to clean, lubricate or even paint a locmotive is that they will be able to take it apart, but never get it back together right again.  This fear should be put to rest as with anything it is all in the technique. 

First, as with any project have the proper tools, a knife is not a screwdriver, just as sissors is not a pair of pliers.  Get a nice set of jewelers screwdrivers and nut drivers.  they usually come in nice organizing boxes that keep them clean and where you can find them again!

Second, save up some egg cartons as they make life so much easier.  While disassembling anything there is always a right side and a left side.  By arranging the parts in order lengthwise in the egg carton that way parts will not get mixed up and it keeps them from rolling off the table and getting lost.  I like to actually mark the carton with a L and R so I don't get it mixed up. Get a get a nice soft piece of foam, about an inch thick and 12 inches square.  This will help cushion the piece you are working on and keep from damaging any delicate parts or scratching the paint.  Besides the foam is nice to stick the screw drivers in while you are working. 

Third make drawings or write out how you took apart the more complicated parts, ie:  Wheel, washer, connecting rod, washer, drive rod, washer, screw.  That way you will know the order it all goes back in when you reassemble.  Make drawings or measurements of small screws as well so that you know where each one goes.  I think everyone has heard of the mechanic that puts an engine back together and has extra parts!  Don't be that guy, every part has a purpose.

OK, now you have it disassembled.  How do I clean it.  I use denatured alochol and a swab to clean old oil and grease out of bearings and off of drive wheels.  It is a good cleaner and leaves no residue.  Once you have it cleaned be sure to have a good compatable oil ready to relube it.  Make sure the oil is plastic compatable if using it on plastics or you will have a soft mush after while.

When reassembling be sure to snug up the screws but don't over tighten or you will strip screw threads.  Snug is fine.  ONLY on drivers do I use nut drivers or screw drivers to put a little tigher torque than the other screws so they will not loosen and come out. 

Do not use the wrong screws in the wrong holes.  If you lose a screw, write who made it and screws are usually available.  A trip to the hobby shop can sometimes work too.  I belive that most screws are still available from North West Short Line and even PFM as well as other current brass importers. 

In any case, take your time, be careful, and don't give up.  If you get frustrated, put it aside for a while and come back to it. 

A couple of extra hints based on experience, don't disassemble over a shag rug.  Also If you have a watch makers apron that slips over your neck and makes a catch basket as the end is put in a drawer in front of you or is attached via magnets to a metal bar in front of you, it saves countless hours hunting for a wayward screw that dropped and bounced at least 100 feet away!

Good Luck