My most successful position is Kimura to back take to attempted RNC. However, i am continuously failing to finish the RNC even though i am not too bad at isolating one arm. Opponents are great at locking down their neck and preventing me from sliding my hand under, forcing me to do a cross face which i don't like doing to training partners. I find higher belts when having me in that position, tend to not waste time and either do the cross face or drill a hand into forcefully into my neck to open up it or just make me tap. Should I just copy them even though it feels like a dirty move?
I used to have the same problem. Be more aggressive with attacking the neck. Don't be so nice. Learn John Danahers method cutting in for an rnc. Also, check out Marcelo Garcia's method of prying of the jaw.
Try using your fingers along the gap where their jaw connects with their chest. You can either use your finger tips or the 2nd knuckle. I prefer to use the 2nd knuckle so people don't grab my fingers, but either way will work. Don't try to do it near their chin. Start at the jaw below their ear and go around. If you feel like you can't push it all the way through, walk your hand around like an inch worm.
Watch this if it isn't clear Danaher RNC
There are other ways, but that is the easiest, most effective and least dirty.
Back finishes are a fundamental skill and basically something you can improve from white belt to black belt and beyond. There are so many options and strategies, given enough time I'm sure you will find many ways to get RNC's on everyone.
Probably my first option is to go to "strong side", using my topside leg as a butterfly hook instead of a back hook to keep them on their side and prevent them from using any bridging escapes. This is relatively secure control for me, and I actually prefer it over the body triangle in most cases (it depends on my opponent's escape style though). Going to the "strong side" tends to work best against someone who is willing to expose themselves a lot to escape (ie, lots of bridge attempts with an open neck, lots of hand usage to pry my legs away, etc). For me, the key details to finishing from this position are finding ways to prevent them from getting a strong defensive grip on my choking hand; ie, hiding it behind their back prior to shooting the choke, or anchoring my hand on their far tricep to make it hard for them to pry it off, or once I can wrap their neck making sure to cover my wrist with my chin, etc.
If I find my strong side is not working well, then I may try to transition to the "weak side" and attempt to trap an arm. Once an arm is trapped, prying their jaw open is a good, classic technique. Personally, I have trouble with it when the opponent outweighs me a lot, in which case I often use a Marcelo-style head forehead pry technique. It's a hard technique to learn but if the opponent is a lot bigger it might be one of the only options. (Transitioning from a "weak side" trapped arm position to an armbar or back triangle are other classic options; personally I've never gotten them to work well for me.)
When all of that isn't working, I may have to give up back control if all I care about is the submission. Personally, I like to secure the claw grip with my "under hook" arm and try to mount. Arm triangle is the canonical attack, but I prefer to advance to an S-mount armbar instead. Also, FYI, I've seen Craig advocate use of the claw grip and a same-side back hook to force the opponent belly-down, under the philosophy that when they use their hands to build base on the mat they will have to expose their neck; I've been playing with this for a few months but it's a work in progress.
Finally, another option is to let my opponent out of my back hooks but keep my seatbelt grip. This sacrifices control for opportunitiy; people are a lot more willing to open up their chin and use their hands for something other than blocking my choke during fast scrambles.
P.S. Here is an example of Craig's technique:
Craig Jones - why belly down back control (starts at 4:01)
Craig Jones - belly down back control (starts at 8:47)
Personally, I work the Craig Jones method to move uke to his stomach. Then I pick the side of the neck I'm going to attack. If its the right side, I cross over his head with my left elbow and place it on the mat beside his ear. Once there I drive that left-elbow counter clockwise to my left hip. This opens up the space between his jaw and neck. Then, because he made me work so hard, I use the knuckle worm method to drill under the jaw line with my right hand. It works extremely well.
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I noticed I finish more when I focus on rotational finishes rather than squeezing.
Yes yes yes to the rotational component. Wiggle your shoulders rapidly and your arm will find its way in.
Also, in like to choke with something like a seatbelt grip so I can do a sneaky switch off if the hand I put in first is being pulled away, the second dives under and deeper.
I have always found that landmarks were required ie choking arm should have your elbow in line with his chin and sternum.
The arm applying the choke should have that hand cupped on either your shoulder or his, preferably your shoulder.