Talk to Your Players
First of all, you have to make sure that they're even in on WANTING it to feature long cinematic descriptions. Some players are more interested in the tactical aspects of combat, or not interested in combat at all and want to just get to the story parts quickly. A popular paper on (video) game design defines a series of different aesthetics/sources of fun, each of which will be more or less important to any individual player (Extra Credits did an episode on that, too, if you prefer watching to reading). Maybe those descriptions just aren't fun to some of your players.
Lead by Example
Once you've agreed that detailed descriptions of actions should be part of your game, lead by example. Consistently describe your actions in the desired detail. Eventually your players will likely follow suit. Partly, because they'll be reminded of your agreement, partly because humans tend to mirror other humans, partly because if they like it from you they'll probably aspire to do their best, too.
Remind Them (at Appropriate Times!)
If you notice that they're falling back to old habits, remind them of going into a bit more detail. But do so at the right time. Do not interrupt play to enforce it. This would be more effective for the learning progress, but it will hinder the game and getting lectured detracts from everyone's fun. I doubt that's worth it. Therefore, do so in a break. Or before the next session. Or in between sessions if you have some form of "debriefing".
Do NOT Introduce In-Game Punishments or Rewards
This is part of the "acting" part of RPGs, not the mechanical game side. Just as I'd advice against punishing or rewarding excellent acting in social interactions, I'd advice against it here. Doing such, gives player skills an influence on character skills where it shouldn't. You're roleplaying after all. There's no reason why the shy guy playing the diplomatically skilled orator should have any disadvantages from not being as good a speaker as his character. Nor should the fighter hit any worse because his player is not all that creative when it comes to visceral descriptions. The Angry GM touches on this here and explains the difference between role-playing and acting here.
And last but not least: Be Patient. They'll get there, eventually.