Bass is often the hardest thing to get right due to the speakers and room you're working in. Having a good playback system with speakers that can reproduce that low end accurately is critical to getting it to sound good. A treated room really helps too.

Floyd also hit on something that not many people are aware of. It is a great idea to high pass the bass. Just like you should do with most everything else, the bass can often benefit from a high pass filter set to around 100hz. You might have to experiment a bit on the slope and the exact freq, could be lower or higher, but it's something you should try. Most people do the exact opposite and boost the low end of the bass thinking it will get them a fatter sound when the opposite is in fact true.

Keep in mind too that you're comparing your mix to that of a professional who knows what to do and has the million dollar studio and knows how to use it. Not saying you can't get a great bass, because you can, but you might not get it from a real track. As great as the RT's are, sometimes you just have to do something else to get what you want. I've been known to use a sampled bass with midi and play my own bass lines from time to time. You can often get a much cleaner bass which has nice definition in the sound and cool characteristics and overtones that might not be in a RT.

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