Finishing from mount consistently is one of the hardest things to do in BJJ. Mostly because it's so hard to maintain the position while attacking at the same time. But the top guy has a lot of small things he can do to keep the position while attacking and if you get good at it it's also the most effective attacking position in the gi. Simple rule: if uke is crossing his arms to defend the choke, look for armbars. If he's not, persist with the choke. Try to get as high as possible in the mount and get your feet on uke's hips before attacking (if uke is keeping his elbows low to defend, press hard on his shoulders to force his elbows up and then slide your knees up). Once you are high and attacking, set your first choking hand as deep as possible. Don't rush above all else, this can take awhile and require several resets. If at any time uke's defense leads to one his elbows coming to the inside of his body (i.e. breaking a plane parallel to his center line) quickly slide your knee up to his head on that side to trap his arm and start working armbars, or transition to s mount. Learning how to use the choke threat to create chances to move to an arm barring position is crucial.
All this is hard to explain. It's a complex position, just don't be in too big a hurry to finish such that you lose position.