Mirror lock up is said to be important for exposures ~ 1/15 second. Markins have an interesting document that shows vibrations as a function of time with and without MLU:
http://markins.com/charlie/report4e6.pdf
It's a bit of a sales pitch for Markins products, but still of interest.
In the past I've always used the self timer and often placed a bean bag on top of the camera. Even at 1/100 second with a 200mm lens it made a significant difference. My new Nikon camera has a mirror lock up feature, but sadly it cannot be combined with the self timer. You have to set it to MLU mode, and then use an electronic cable release. I've had one on order from Nikon for 6 weeks so hopefully it will arrive for Christmas.
Another trick for longer exposures with flash is to use rear curtain synch so that the flash fires after the mirror and shutter vibrations have had a chance to die down.
BTW if anyone knows where to get Nikon accessories without a 5+ week wait please let me know. I've an ES-1, 3T and MC-30 on order and have been waiting a couple of months in one case.
Leif