I primarily work on doom-ier type material and I do find it difficult sometimes to fill out the sound. Here are some ideas/things I do: Use a transient designer to pump up the sustain on the kick so that it can fit out the drum parts more. Use huge (not big, but huge) plate reverb on the snare and toms. Distorting the fuck out of the bass track (not like the blend method, I mean making the entire bass filthy-nasty with a nice sub low end) will give more sustain and will fill up the sound (more harmonics). Sometimes mixing the bass lower than the kick (giving it a nice 50-60hz solid low end) helps because the bass is sustained and the kick is usually spaced out more in the arrangement.
I also find that the arrangement is crucial for two reasons: One, I think that sometimes doomy mixes sound a little too open and empty because there is not enough going on to fill up the wall of sound. Depending on the type of doom you're after, you may find it helpful to add some extra guitar layers or instrumentation on top of your main rhythm guitars. For general melodic doom, synth pads that follow the chord progression add thickness to the guitar high end. For funeral doom, string and brass movement complements the long sustaining guitar chords well. For stoner/occult doom, vintage keys can help thicken up the center; some kind of organ or mellotron mixed in to support the guitar rather than stand out can really add that midrange weight.
Two, dynamics are so damn important. Having breaks between crushing riffs or lead-ins that build up the layers of heaviness cannot be overlooked. Even though it's just simple psycho-acoustics, a riff will sound so much heavier when contrasted against a single guitar line lead-in or a bass/drum breakdown that provides a dynamic break from the riffing. Even just having the drums solo for a short fill before busting into a crushing riff can be enough of a dynamic break to help give a renewed sense of heaviness.
I hope my longwinded reply makes sense and maybe helps a bit.
