Don't Forget Your Air

Sometimes it's easy to think that articulation is something separate from air. But actually, when experiencing a difficulty in articulation, check and work with the air first. Very often, we actually focus too much on the tongue (tonguing too hard, tongue feels "tense" or slow) and not enough on the air. Accents, for example, should be backed up by the abdomen and not from the tongue or even embouchure alone. Otherwise the sound is killed. We end up having a lot of tongue and little remains of the sound. This can also cause slowing down in a passage. 

One also doesn't do much favor by "kissing" the sound - keep lips in place and work with the breath. Look in the mirror and one might be surprised.

Articulation is dependent on air, just like photography is dependent on light.