When soldering standard sterling silver, brass, copper, nickel, bronze, gold or silver-filled pieces you will want to use two types of flux. One flux to protect the metal from firescale and another to help the solder flow. Our Jewelry Tech Team calls these a "barrier flux" and a "flow flux."
There are several barrier fluxes on the market that you have to spray onto a piece, like Cupronil, Stop Ox, or Firescoff. Another alternative is to use one that you dip into or paint on. This second type is a boric acid powder/denatured (or methyl) alcohol mix. It's an old recipe that has been around for a very long time and it works great to protect the metal from oxidizing during soldering. It's homemade and easy to mix.
Pour the alcohol into the jar. An inch or two is really all you need in most cases.
Next, add the boric acid until you can see that the ratio of powder to alcohol is about 50/50 (this is why using glass is handy). A 50/50 ratio is just a standard recommendation—some like to use less boric acid, some like more. I try to keep the acid at no more than 60%.
To use the mix, you can either dip the piece into the solution or paint the solution on. I highly recommend painting small pieces like jump rings, since they're easy to drop into the solution but nearly impossible to retrieve! If you dip, then be sure to swirl and mix the solution; otherwise you won't get a good coating.
Once the mix is applied, you can either wait for the alcohol to burn off or light it and let it burn off. I like burning it off because it's quicker—and it makes a pretty blue/green flame! You should be left with a nice, even, powdery coating.
Now you are ready to apply the flow flux and the solder and do the soldering.
Flow fluxes include Handy Flux, My-T-Flux, Rio Ready-Flux, and Grifflux among others. These make horrible barrier fluxes and don't protect the surface very well. They typically bead up and give you a polkadot firescale pattern. Which could be cool if that's the kind of thing you are looking for—but normally it's not!