Compression
Seriously, unless you are going for a specific effect, I would use the Chorus/Doubling etc as a LAST RESORT ONLY!
Wierd phasing things can happen here, and make it sound thick, but often incredibly unnatural, unless it's VERY carefully applied, with much care and attention to phasing at different frequencies.
My first question to you would be "How much compression do you use"? Make that compressor work for you.
If you have an opto compressor, this will be best, as it will add a natural "fattening". Set the MAKE-UP gain up around 3-6dB, then use a 3.5:1 or even 4:1 setting, and keep it contained.
This in my experience is far more helpful in getting it thick without making it sound completely unnatural.
EQ a LITTLE bit, maybe pull out 2.5kHz around 2-3dB, use a Q of about .3 (point three).
Trust me, with you can setup a compressor to the point where it can completely dominate the mix! They are marvelous tools, but must be used judiciously.
I generally avoid the chorus/time shifting techniques at all costs, as it's more of a make-shift bandaid, which doesn't come without it's penalities.
Cheers and good luck!
SH