Congratulations - you're asking a fantastic question.
I think there are a few answers to this, and the most relevant ones probably vary at different points of your playing development.
The two most basic ones are:
1. You always have to keep a great conception of sound at the forefront of your mind. I think it can be quite difficult to really sound great in the range right around middle C on the bass trombone, and I think this register is key. You have to make sure you have a solid conception of a great, centered, easy sound in that range, and then carry the ease up to the higher register and the center down to the low. It's very easy to fall into the parallel traps of forcing the high register and letting the low register be flabby.
2. You have to do things - exercises, etudes, etc. - that have you moving between registers, and the most fundamental traditional thing to do is lip slurs. I've been rediscovering lip slurs recently, and really seeing the benefits again. I think one point to make about slurs is that slow ones are important, but so are fast ones - as long as you keep the conception of sound and really listen for quality. Check out Brad Edwards' recent book of lip slurs, and also Paul Faulise's book of exercises for bass trombone. Faulise extends the lip slurs through the valve register - actually playing lip slurs covering many partials with one and two valves down, and then going down even lower into the pedal register. This is great stuff, and really helps develop sound and ease of production all over.
One more thing: For me, it is very important to practice tonguing, always striving for clear, clean articulations in every register. I do a lot of marcato tonguing in all registers, with very long notes - essentially playing legato but with a marcato tongue. I also very much like practicing triple tonguing, striving to do it clearly in all registers. I find that these things help promote very centered, very efficient tone production and help me to project a solid sound without having to play so loud that I cover other instrumental voices.
The question you're asking is really a lifelong project. If you're anything like me it's something you'll be trying to make better and better for your whole life. I feel like I've only recently started to get to where I'm happy with this aspect of my playing the majority of the time. And I know I'll never be done making it better.