Depending upon the look you are trying to achieve you may or may not wish to add polish to your work. If you want it to look matte, then just leave it as is, or polish it on a piece of rough cloth. For a subtle polished look you can use 'Armorall', or a book restoring wax called 'Renaissance Wax' (bookbinders may stock it). These are applied, then polished with a soft cloth. In the US a floor polish called Future (in Australia marketed as Pledge One-Go) is used by many polyclayers - a few coats gives a nice sheen.
Fimo, Sculpey & Lisa Pavelka make a gloss/satin varnishes specifically designed for polymer clay. When varnishing beads it is a good idea to suspend bead on a toothpick into it's hold & then stick this into a wad of scrap clay to dry. One not too thick coat is enough usually, however for items which may be subjected to a lot of wear, 2 coats is advisable. Varnish is essential to seal clay which has surface embellishments such as Pearlex metallic powder, metal foils, or other material which may wear off over time.
Pearl Ex varnish is also good to use over Pearl Ex Mica powders to prevent them wearing off, and can be used on just polymer clay as well.
Beware of using other brands of varnish which may not be compatible with polymer clay as they can react with it and go very sticky over time.
Sanding & Polishing:
For a lovely mirror shine that is natural you can consider sanding & polishing. It is quite labour intensive but in my opinion work the results when you are making a special bead or pendant.
When I want a really smooth finish I first sand with wet and dry sandpaper. Get a bowl of water and start with 400 grit then move to 600, 800, then 1000 or 1200. You can stop there and just finish off on a piece of rough denim or cloth, or if you want a really high shine, use a muslin buff on a buffing wheel or a tiny muslin buff on a dremel craft drill. This brings up the clay to a mirror finish. This finish is more natural and attractive than varnish, however is not always possible if your piece is rough in texture or has surface embellishments which you don't want to sand off.
It is a nice professional finish to sign the back of your work - either with a permanent pen, a rubber stamp made from your initials (see the sets available on office supply stores & mount glue onto a block of baked clay for a handle) or make a small signature cane to stick a slice on the back.
Make sure the back of your piece of work looks as good as the front - people love to turn things over and be surprised finding a lovely finish or added embellishment.
Use good quality findings to complement your jewellery.
Coloured "Artistic Craft Wire", Charms, old Watch Parts, and other found objects can be used to add interest and colour to jewellery, stamped pieces of clay, or sculpture.