Here's a hot tip I got from someone.

Crayola makes a clay called Model Magic. This stuff is not too expensive, is very lightweight, and smooths wonderfully.  You can use this for facial features (noses, cheeks, ears) and other  things that because of their small size would be difficult to sand and smooth. It can be used for anything that you don't want to sand because sanding might undo a perfect shape, like domes.

Model Magic is somewhat difficult to work with. It is not truly additive like regular clay. Adding a little more clay to your piece results in seams that can't be eliminated, no matter how long your work it. You must start with the correct amount of clay, then smooth it into a ball, then work it very gently to avoid pulling it too hard causing overlaps, thus retaining it's smoothness. If you find that you don't have enough clay to get the size and shape you wanted, add the clay then start completely over.

This 'clay' has a very long working period.  It takes a full 24 hours to dry, so you can work your piece of clay for up to about 5 hours.

The best things about this stuff is that it is incredibly smooth after it dries, and it is very lightweight. If you don't want to spend time sanding before painting, this stuff is wonderful.  Also, it is very strong and lightweight. You can glue it onto your work and paint it's surface, or layer paper over it and leave it in your work.

The worst things about this stuff is that (1)  folds result in seams that can't be eliminated, and (2) it is not sticky. It will not stick to your work. You must glue it into place after it dries if you want to paint it's surface, or you must let it dry thoroughly before layering over it if you want to use it for the form you're gonna layer over. Also, If you're going to glue it to your piece and paint it's surface, you will find that it shrinks a little, and you won't get smooth seams between it and you paper mache when you glue it on. However, you make make glue paste to fill this seam.

How to make your glue paste and work with it:

Mix white glue (PVA) and wallpaper paste in a 1-1 ratio and mix in ground chalk until it's a thick and fairly stiff consistency. DON'T add paper pulp or it will be lumpy (I hate lumps)!

Apply this liberally to the underside of your Model Magic attachment, then apply pressure to adhere the piece to your paper mache letting the excess squeeze out at the seam. This will form a bead of paste around the connection. Don't remove this bead or play with it while its still wet and gooey or you'll introduce new problems. Wait!

After about half an hour or so, go over the bead  with a squeegee made out of cardboard from a cereal box. Use this squeegee to remove  most of the excess paste, but leave a little to sand away with sand paper (when it dries, the paste shrinks).  When the glue-paste is thouroghly dry, sand the seam with 180-240 grit sand paper (on a a pointed object as the sanding block -- a golf tee works very well for this).  Voila! You'll have a very tight connection that once painted will be hardly noticable, and a very smooth feature that looks like it came off the Pilsbury Doughboy!