A lot of detuning, unison detune, chorus detune etc.
The EQing can also give the synth a darker sound. Lowering certain Mid frequencies so soften the sound and make it sound sharper.
Also using a filter at the right time can give you some pretty cool hard house effects.
Check these out dude: http://www.yousendit.com/download/Q01GUXVtSyt0Ni92Wmc9PQ
Those were done with a fuckload of saws unison, chorus on the sytrus synth in FL. In this example I even carefully automated the unison so that different elements phase throughout the riff, giving it an 'alive' feel.