if you're into breaks, drum 'n bass, hip-hop or many other electronic genres, you will want to deliver a solid low-end - but you may find your bass patches under deliver at these frequencies. enter : the sub-bass.
firstly, be sure you're using a subwoofer - the sub is often going to be more felt than heard, and without a subwoofer you're going to miss out on a lot of what you're working on.
a sine or triangle wave are good choices for sub basses - typically they should follow your bass line, perhaps an octave lower. if you are using one of these waveforms and you notice that the volume seems to change based on which note you're playing, the most likely reason for that is that your sound system is not capable of producing the frequencies you're generating with equal power. also keep in mind that though the human hearing range can go down to 20hz or so, your ability to hear these sounds drops off considerably at these low ranges - assuming your sound system is capable of reproducing the frequencies you're sending at it, you will want to listen with your body more than your ears.
many will talk about the need to compress sub-basses in order to manage them; i would disregard this. if you are using a straight sine or triangle wave and want to alter dynamics, edit the amplitude envelope (probably leave the sustain near max).
also avoid EQing, with the exception of a high-pass filter to eliminate sound energy from inaudible frequencies that will do nothing but eat up headroom. reason's m-class EQ has a low-cut feature for this purpose.
many people recommend using an exciter or distortion unit to add higher harmonics - the idea here is that by adding upper harmonics, you can induce the human ear to perceive the fundamental even if it's outside the audible range or not being produced adequately by the speakers (i.e. for particularly low notes). this is not a misguided idea but a more straightforward approach would be to add those upper harmonics yourself through the use of an additional oscillator - for example, you can add a band-passed saw wave an octave or two above the sub at very attenuated amplitude so that it is barely perceivable. this will allow the listener to perceive the note even if/when the fundamental is too low to be audible or reproduced. that being said, if you've got access to a VST/AU exciter, by all means give it a shot. you may find you have greater control over the signal if you are managing the frequencies at the oscillator level rather than seeing what the plug-ins do for you.
if you're using a sub-bass, it will probably be essential that you employ side-chain compression - your kick and sub are likely going to be fighting for the same frequency range. side-chain compression will get the sub out of the way while the kick is ringing out - this way you can avoid phase interference and preserve the power of both signals.
lastly, while mixing, compare your output to a well-engineered reference track. you may be inclined to try to make your clothes shake as they would in a club at non-club volumes - you can imagine what would happen if you took that mix and actually played it on the big system :)
