I've been playing bass for 9 years now. To be honest with you, I've been practicing almost exclusively my technique for the past 4 years, but now, I've decided to practice my groove chop and I would like to improve my timing.
To improve yourself, there is no secret, you must practice. How? Well, I generaly practice 2 hours per day (some people do more, but for me it's enough). During this 2 hours, I practice with my metronome at 60 bpm playing only 8th. I do a round up of all my techniques (fingerstyle, slap, tapping, pick, flamenco stuff...). The reason why I play only 8th notes this slow is because it's the only way to pay attention to how your fingers work. If you can play a lick perfectly (and without any stress) at a slow speed, there is no reason why you couldn't play it fast. I like this analogy : Did you learned who to run? No, you learned how to walk and then you were able to run. It's the same things with music. So be sure that you are really relaxe when you practice and pay attention to your tone. You can also practice in front of a mirror to see if you are really relaxed or not.
I hope this make any sense.