Here’s a nifty abridged version of some v/o advice and general diction guidelines from a recent Edge Studio newsletter:

Every natural conversation requires a different level of articulation.  It depends upon our listener, our environment, and our intent.  Sometimes we speak formally (enunciate words clearly) – other times we speak informally (colloquially).

The voice-talent’s job is to determine how formally or informally the script should be, and then deliver the script accordingly.  In other words, pronounce words as they are spoken in genuine conversation, and then tailor them to match the formality of the script.

a. Do not over-articulate, or your delivery will sound contrived and
  unnatural.  While it may seem obvious to pronounce every sound very
  clearly, it is more important that the delivery sounds natural.

  Following are words that are often over-articulated:

            effective  (The first “e” should be soft, not hard.)
                often  (The “t” should not be pronounced.)
              vegetable  (The “et” should not be pronounced.)

b. Determine how formal / informal the pronunciation should be, and adjust
  accordingly.  Use your discretion.

  For example, sometimes the letter “t” should be pronounced, and sometimes
  it should not.

  INFORMAL: In the commercial below: do not pronounce the “t” in
  “mountain.”  In the promotional video below, do not pronounce the “t” in
  “out” and the second “t” in “Internet.”

        (commercial) Up here in the mountains, we love Coors Beer.
    (promotional video) So check us out – we offer the fastest way to the
                                  Internet!

  FORMAL: In the commercial below, pronounce the “t” in “commitment” and
  “Vincent.”  In the documentary below, pronounce the “t” in “mountain.”

  (commercial) Service and commitment…what you expect from St. Vincent
                                  Hospital.
      (documentary) The Atlas Mountains stand over 13,000 feet high.

c. Words with multiple pronunciations often occur in scripts.  When
  deciding which pronunciation to use, the rule of thumb is to use the
  dictionary’s first phonetic pronunciation, as that is the way the
  majority of people pronounce it.  Here are examples:

        interesting: pronounce “in-trest-ing”, not “in-ter-est-ing”
          comfortable: pronounce “comfter-ble”, not “com-fort-able”
              February: pronounce “feb-u-ary”, not “feb-ru-ary”

  * TIP *  Sometimes different pronunciations of a word are equally
  acceptable.  In these cases, the producer has final say of which version
  will be used. Sometimes, the producer may ask to hear both versions to
  help them choose.  Once decided, it is necessary to remain consistent
  throughout the entire script (in other words, pronounce the word the same
  way each time).  Following are examples.

        finances: fuh-naan’-sehs, phi-naan’-sehs, or phi’-naan-sehs
                    details: dih-tayls’, or dee’-tayls
                        data: day’-teh, or dah’-teh
                  envelope: ehn’-veh-lohp, or ahn’-veh-lohp

d. The use of contractions is generally preferred in informal scripts.

        For example:  You know when it is hot…I grab a cold 7UP.

  …should be read like this:  Ya’ know when it’s hot, I grab a cold 7UP.

e. Lazy mouth is a vocal occurrence where undesired sounds are enunciated
  before “hard” consonants.  For example, the “mmm” sound is often
  vocalized before the word “bye.”  (If you haven’t noticed people saying
  “mmm-bye”, pay attention for it…you’ll be sure to hear it.)

  Unfortunately, while lazy mouth is a very common, it is usually not
  preferred by producers.  Therefore learning how to rid yourself of it is
  smart.

  There are 3 common occurrences of lazy mouth, they are:

    . “m” sounds before words beginning with “b”:
                        say: Brought to you by Aetna.
                  instead of: mmm-Brought to you by Aetna.

    . “n” sounds before words beginning with “j”
                say: JC Penney introduces their one-day sale.
          instead of: nnn-JC Penny introduces their one-day sale.

    . “n” sounds before words beginning with “d”
            say: Duracell batteries are the most trusted battery.
      instead of: nnn-Duracell batteries are the most trusted battery.

  To rid lazy mouth from your vocal delivery, read one of the above
  exercises while exaggerating lazy mouth (in other words, read the example
  very incorrectly).  While articulating the lazy mouth sound, notice the
  position of your tongue and mouth.  Then read the same example again with
  a smaller occurrence of lazy mouth.  Then read again with no lazy mouth.
  Now you should have the ability to notice when lazy mouth appears in your
  narration, as well as the talent to correct it.

If you want even more information on voice technique, consider purchasing Edge Studio’s Industry Guidebook, which has plenty more tips and training techniques like the ones you see above!
You can order the guidebook online at:
https://www.edgestudio.com/guidebook.htm