Easiest solution for you? Give on-the-spot situational bonuses on the to-hit roll or on damage for a good description (e.g. "I jump off of the hillock and direct a two-handed slam against the Orc's head." "Cool. Roll to attack. You hit him, and +1 damage for a good description. He looks like he's faltering."). Biggest issue with that is that it penalizes players who aren't creative on a spontaneous level and can lead to "I can top that." descriptions that derail the flow of play. One possible solution to that is to only allow one such description in battle, allowing the less improv-happy players to "prepare" such a description in advance. An alternate version is to give some small award after the encounter to everyone who chipped in a good descriptor. Don't make that one a competition though, to avoid the "top that" derailing.
Another potential solution is to raise the general standard of play. Add in your own descriptions to NPC attacks (but don't add the situational bonuses). Those players that you know are good at adding descriptors, encourage them to do so. Often, this causes the other players to step up their game. This is the gentler solution.
Lastly, especially if some players are having issues at coming up with ideas, consider a short workshop session where players chime in on good descriptions based on a general scenario or an image. It helps if you have good and bad examples from literature, RPG sessions, etc.
Of course, one needs to have the additional disclaimer that this may or may not work for any particular group or players. Some players are happier just saying "I roll to do a Standard attack" and forcing them to do otherwise will just cause stress for them, which can lead to stress for you. Consider how much stress you'll take either way and weigh that in your decision.