Also note: Please clean carefully and at your own risk; be especially careful with glittery or fragile symbols. A great cleaning page to try would be the MLPCB Pony Salon,  which covers Rainbow Curl Ponies, So Soft cleaning, smooze, rust, defrizzing and much more. The information below is very basic and brief.

1.)  Bodies of Regular ponies (not so-softs):

    For simple cleaning (when the pony is simply dirty) use non acetone nail polish remover.  Soap and water work, but it often doesn't get the pony clean enough.  Some people also suggest comet.  Stay away from the symbols with the nail polish remover, and all other abrasives.  It will rub them off!  Cotton balls can be used for large areas, and Q-Tips work well in smaller areas.  Be careful with the Beddy-Bye Eye Babies, try to keep any liquids out of the eyes because of the risk of rusting.  Also, watch jeweled eyed ponies, they will scratch. Also Plush eyes are also prone to scratching.  
    Here's the remedy for age spots.  Place Elmer's glue over affected spot, let dry and then peel.  I have never tried this so I can not tell you how well it works.  However, it is noted that this can cause a slight discoloration on the pony. 
    For sticky messes, both something like go-jo (orange cleaner) and nail polish remover work.  Again, stay away from the symbols.  I have even heard of people using 409.

2.)  Bodies of So Soft Ponies:

    Much soap and water.  We gently scrub the pony's body with a soft toothbrush dipped in soap and water. This helps get dirt out and any lint that may be stuck in the fuzz.  In the case of white so softs, try using a very small amount of nail polish remover to remove extremely stubborn marks.   I have used a soft-bristled toothbrush before, but be careful or you will lose flocking.
   Do not try to bleach white so-softs; it doesn't work.

    I also suggest Oxyclean; I have used it on my So Softs and it seems to work great!  Just soak in Oxyclean and warm water for about an hour.  

3.) Pony manes and tails

     For hair, use a small amount of shampoo, then conditioner.  Some suggest No More Tangles (Johnson and Johnson--I think). Let the conditioner sit in the hair for a couple of minutes (or however long it takes us to get through the bunch).  I comb through mine while the conditioner is still in.  If a pony's hair is really frizzy, use a bit of gel.  For curl, use a pencil or the like, regular rollers also work.  Do not apply heat, such as a curling iron to hair.   I wouldn't suggest brushing hair that still has the factory curl still in it.

4.)  Playsets and accessories

   There isn't much to this.  Bathtubs are helpful.  Shower sprayers are great tools.  Toothbrushes are good for detailed spots, and nail polish remover can be used for spots.  Lots of manual labor.