I believe Tizanidine (Zanaflex) works through a different pathway in the brain than Soma and/or Flexiril. I have been on just about every RX muscle relaxer, including Flexiril (felt like a Placebo to me) & Soma (this did help somewhat).
Tizanidine, however, has been the best for my muscle spasms to date. YMMV of course, & it is technically an antispasmodic as opposed to purely a muscle relaxant. I did not have as much luck with the capsule form of it as I have had with the tablet form, and from what I've heard, many other patients also prefer the tablets.
If you haven't tried it yet, I think it is definitely worth at least asking your doctor about.
Also, I have been Rx'ed Diazepam (Valium) before for my muscle spasms. It did work well for them and there are some that consider it the 'best' muscle relaxer, period. I had to get off of it, however, as it began to make me feel terribly depressed after a little while. Not everyone has this side effect, but it is one possibility to watch out for.
Best of luck!