The most important thing to remember about pronouncing the Spanish b and v is that in standard Spanish they are pronounced exactly alike. Although English makes a clear distinction in how the two letters are pronounced, Spanish does not. The sound of the English "v" such as in the word "victory" does not exist in standard Spanish.
The sound of the letters varies, however, depending on the sounds around them. Most of the time, the b and v are what are called voiced fricatives — in this case, a sound somewhat like the English "v" but with the two lips touching instead of the lower lip and upper teeth. Think of it something like the English "b" but quite a bit softer.
When the b or v comes at the beginning of a word or phrase, that is, when spoken after a pause, the sound becomes more like the English "b." This also holds true when the b or v comes after an n or m (which in that case both have a sound similar to the English "m"). However, the Spanish b or v sound in such cases is not as explosive as the English sound; in other words, it is softer.
Because the v and b sound alike, spelling problems with these two letters are very common among native Spanish speakers. And a few words — one of them being ceviche or cebiche, a type of seafood dish — can be spelled with either letter.
When spelling out loud in Spanish, the b is sometimes referred to as be alta, be grande or be larga in order to distinguish it from the v, sometimes called uve (which became its official name a few years ago), ve baja, ve chica or ve corta.
Words and phrases spoken by native speakers in the accompanying brief audio lesson on b and v are buenos días (good morning), centavos (cents) and trabajar (to work).
Final note: Over the years, I have received occasional emails from people who tell me that they have noticed some native speakers pronouncing the b and v differently (not as in English, though, but differently from each other). I don't doubt that under some circumstances this is true; there very well be some areas of relative linguistic isolation where past distinctions still exist, or perhaps where some speakers have adopted them from indigenous languages. But any distinction between the two letters is the exception rather than the rule, and if you follow the rules of pronunciation given in this lesson you will not be misunderstood.