Spotify, itunes and youtube have started to normalise all sound so achieving the loudest will soon be a thing of the past and will actually take a lot of dynamics out, still, achieve a good standard of peak volume. But smooth has answered this, sample selection, the mix, processing sounds individually and by bus, if you do all this all it takes is a limiter on the final output or compressor and youve pretty much got a solid mixed and loud track. I put eq meters and other meters on the output both before the limiter/processing and after. If your really stuck for any ideas on what to do, id start by processing your drums bus with some nice eq plugins like vintage stuff, (T-racks do great plugins), this is my personal taste but i does give some nice colours and fatness to the sound, You can compress or limit aswell. Make sure that you dont have unwanted frequencies on samples and sounds, Im working on something that doesnt have any cuts or ducks in the mid freq or barely any, if you choose the right sounds to start with, your not going to have to do as much crap to the sound to make it fit or sound right. But it all depends on the style of music your going for. Hope this is not to much of a scatty respone haha
I've started using a limiter from the start on the master and turn it down to -6or7, but as mentioned it starts in the mix down, layer drum sounds, make sure nothing is competing for room on your analyzer, (or sounds like it for that matter) push things till the sound like shit, but watch your meters on plugs and what not. limiters are good but the harder one works the further distorted your sound becomes, (Try using more than one to share the work load after your mix down sounds as good as you can). Youtube would be the short answer lolz
However, if you must:
The most important factor is still to have a balanced mix. You won't reach "peak loudness" if your kickdrum sits 6db above everything else, your pads make unnecessary low frequency rumble and so on and so on.
If your mix is not up to standard, no limiter in the world can help you.
Check your mixdown against a reference track that sounds like you want to sound (or at least very similar). Use your ears, use your spectrum analyzer. Compare, write down the most obvious differences between where you are and where you want to be, fix that, compare again. Repeat until done.
But again, at some point you will have to start doing stuff that should make you think. For instance, bass takes up a lot of headroom, which is bad for loudness. So at some point you will have to prioritize between loudness and bass. And if you're making drum & bass, that should make you pause for a minute and think about what you're doing.
I used to think it takes a lot of killing peaks etc, but I've found it's not about that that much (for that, I use saturation; I never limit any channels).
It's like Lug00ber said: it's about having a balanced mix.
If you have sounds fighting for space, it's harder to get clean and loud limiting, and it'll end up sounding like a loud mess at best, but if everyone sits nicely in their own spot, you're well on your way to getting a loud, clean master that only needs a little master polish.