First, get it right at the source, as Dan said.
Second, edit it to fix any imperfections that may hinder you later when you mix : you may use pitch correction, slip editing and/or volume automation, according to the problems you hear. You need to get it perfectly in tune, sitting right after the kick drum (make sure you edited your drums PRIOR to tracking the bass !), and with a volume as even as possible.
Third, process it and mix it appropriately. Like many guys on this forum, I will recommend using a DI track for the lows, and one or several amp(s)/ampsim(s)/pedal(s) for the rest of the spectrum. Filter them appropriately, then EQ and compress them. With your EQ you will have to check that the overall frequency balance resembles the sound you want, and fits the rest of your mix. In particular, make sure that :
- the bass doesn't conflict with the kick drum's main "thump" frequency (usually 70-80 Hz)
- there's no excessively resonant notes or dead spots in the low end, which often happens on cheaper instruments (beware, you can't always fix dead spots by boosting, because EQing can't add frequencies that aren't already there !)
- there's no "farting" due to distortion (this can be prevented by highpassing distorted tracks and, if necessary, rolling off the lows BEFORE distorting them)
- there's no excess in the low mids (most raw bass sounds contain WAY too much shit in the 200-600 Hz range).
With your compression, you will have to smash the bass (either the full instrument, or just its low DI track) to hell and back so that its level barely moves, to get that even layer of low-end that characterizes many professional mixes. Chaining several compressors is common practice, and the task will be easier if the DI track's volume is consistent, so that the compressors only have to control the envelopes instead of fixing inconsistent playing.
Last, make sure there's room for that solid low-end in your mix. Your kick drum has to be EQed so that its low "thump" AND the bass guitar can be heard clearly at the same time. This is usually done by giving the kick a couple of strategic cuts, so that the bass can dominate in the 120-250 Hz region (or so) and the bass/guitar crossover can happen unobstructed right above this. Likewise, you will need a highpass filter on your distorted guitars, to prevent their low-end rumble from messing with your bass.
This is only standard advice, but it may be good for newcomers to have it summed up somewhere
