A huge part of ad libbing is pacing yourself. We often get caught up in all of the 'tricks' and 'trills' that are possible that we get tripped up. Once you you have developed an ear for adlibbing you then need to learn to 'map' your vocals. You  can't do every possible trill each time it comes to your head. Remember that the melody always comes first, and the song must remain recognizable at all times. You have to have a plan to return to the melody at least half the time to give people a feel for the 'center' of a song, otherwise they can't follow you.That may mean altering every other phrase or one out of every four phrases.

When intensity builds its not wise to try to show intensity by doing more runs; Intensity = power. Think 'power' first, embellishments last. If you can't sing a high note exactly like the original artist, you have to show intensity soon after on a lower note of a climactic part to make up for it. This way there isn't a complete 'drop'.

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1. Add/ embellish one note at a time- take the song one phrase, one section at a time and once you’ve learned it phrase for phrase, experiment by adding a note or two to add color. You may find that you need less than you think for successful adlibbing.
2. Focus on one vocal line to embellish- find your ‘sweet spot’ in the hook of the song and sing along with that. once you have figured which part is most comfortable (melody or harmony) embellish those.
3. Memorize chord structure to anticipate more complex or transitional adlibs.  Once you’ve understood your phrasing, you can play with a transition, say from verse to hook or hook to bridge and find ways to link them vocally.

The main goal is to sound ‘clean’... No need to slide all over the scale to show that you can sing. Precision is much more important.