Firstly, you have to realize that loudness is genre dependent and not every track needs to sound loud and thick. Obviously there are 2 stages when you can make your track sound loud.
Mixing stage and mastering stage
It is always good idea to have your song mastered in a professional mastering studio. The most prominent and successful mastering engineers have years of experience, trained ears and necessary knowledge how to use the sophisticated mastering processing gear. Mastering is very important final step, and any respectable mastering engineer will tell you that even a professional mastering studio with extremely skilled engineers and the best equipment and facilities won’t be able to make a bad mix sound really loud and clean. Therefor firstly you have to learn to cover the ground on the mixdown department, so the mastering can take your mix to totally another level.
Using quality samples and recordings
You have to have a decent listening environment, so you can make decision what is a good quality and what isn’t. Then you will easily learn to know a good quality sound when you hear one. It is all about training you ears – daily practice will get you there. When you hear sound, tend to listen for the low and high ends especially – do they sound clean and crisp? Are there any unwanted noises? Another important thing to listen to is the dynamics. Are they intact?
Learn compression and don’t ruin your tracks with too much limiting
People often think that compression is a magic pill that will help sound everything good and loud. Sure, compression is a great tool, but you have to learn how to use it properly. General rule of any compression is to use low ratios and to avoid short attack times. Short attack time usually kill audio. To avoid over-compressing your mixes you should remember, that the higher the ratio, the higher the threshold should be. Never forget, that heavy compression is always audible. Better idea is to use compression gently at different stages of recording and mixing. Also if you want to go for loudness, it is beneficial to learn how multiband and parallel compression works.
Saturation is very powerful and often overlooked tool. By adding little bit of saturation to individual channels you can drive your mix a tiny bit louder. It is always worthy to experiment with saturation – different plugins give you different results. You can add saturation to bass track, tape saturation work excellent for drums. Saturation in any form adds harmonics to the sound. The perceived volume is raised as natural compression with little bit of limiting. These combined effects add up to what could be called ‘fatness’ or a ‘warm’ analogue feel. Remember that saturated audio should still sound clean and not distorted.
Be careful with bass
Bass is usually taking lot of energy in the mix. It is always good idea to use low-cut filter to stop very low frequencies to get out of control. If you want to be sure what’s going on there, use a spectrum analyzer!
Balance your EQ
If your mix is unbalanced, it will never sound loud. In other words – balance is everything. For total loudness of your mix is very important to have balanced your Mids and Hi-Mids area. Our ear hearing is very sensitive in the 2kHz region – you can always use this trick to make guitars and drums sound loud and aggressive without pushing up the average level.
At the end of the day don’t worry about loudness too much. As you will practice with mixing and as your ears will be more trained, your mixdowns will get better and louder as a byproduct. Also with more practice you will develop your musical instincts: you will be able to trust your intuition while making a decisions.