A lot of the time the preamp used in the recording phase can be the problem- especially with samplers. Record into your sampler with a tube pre (even a cheap one) and follow it with some eq.
For synth pads, a nice thickener is recording the sound, copying it twice, low-pass one, bandpass the other, and then layer them in your sequencing software. I do that and then play with the panning on them, just gives the pad a nice meatiness.