You need to get a metronome and practice with it. Do not try to play your bass while you practice this, not at first. Start slow and increase speed as you improve - start at like 50 beats per minute. (this is harder than you think really slow!) Bob your head in time with the metronome. When you are able to "lock" your head bobs with the metronome's clicks, try changing it up a bit. Try two bobs per beat. Next, rather than bobbing and recoiling with each beat, bob your head forward on beats 1 and 3 and recoil back on 2 and 4. When you do that you look like you're 'really feeling it'. Once you have these basic variations down and you can change back and forth fluidly, and you have worked up to practicing at about 120 beats per minute, then you need to work in your rock faces. Start with the basics like the standard rock sneer and the ever popular pooched lips looks and work toward more advanced rock faces like tongue out snarls or the ladies favorites, the tongue out tongue wiggle or the always sexy lip and teeth licks. Learn how to work the more advanced rock faces into a good solid rocking out regimen. Obviously you can not stand on stage and lick your lips all night, you'd look like a moron. But if your are rocking out hard doing a good solid "lost in thought" look type head bang and then you go into a bad ass snarl and follow it up with a well executed lip lick - well now you've got something! When you are able to head bang and make rock faces in time with your metronome and go through all of your head bang and rock face variations, then you are ready to pick up your bass and try playing while rocking out. If you can't get your playing to match tempos with your well rehearsed rocking out you need to adjust your amp - try lowering the mids so that you don't cut through the mix quite so much and lower the master volume. I'm sure nobody will notice your bad timing if they can't hear you. Just make sure that you look good on stage and are rocking out in time with the band. One the other hand, if you insist on making your bass playing the top priority I suggest following Munji's advice.