there are waves plugins that made a sort of M/S but I read they don't work very well...but you can do the same thing manually.
You can get M and S from a stereo file with those equations:
M = 0.5 * ( L + R )
S = 0.5 * ( L - R )
therefore, M is the mix of L and R, 6dB lowered and S is the mix of L and R (R with inverted phase), 6dB lowered.
When you have setted all your things with M/S, you can obtain the L/R image with those equations:
L = M + S
R = M - S
L will be the mix of elaborated M and S
R will be the mix of elaborated M and S (S with inverted phase).
With you daw, you should create 2 tracks (one for the Left channel and one for the Right channel). Now, you have to copy those 2 tracks in 2 others tracks.
1 -> L
2 -> R
3 -> L
4 -> R
Now you can rename the tracks this way:
MIX L1
MIX R1
MIX L2
MIX R2
you have to set the level of the 4 tracks at -6dB and MIX R2 have to be phase inverted.
Made this, you have to create 4 mono groups:
GRP M
GRP S
GRP M2
GRP S2
Now:
---> = send
MIX L1 and MIX R1 --> GRP M
MIX L2 and MIX R2 --> GRP S
GRP M --> GRP M2
GRP S --> GRP S2
with send level setted to 0dB
GRP S2 have to be phase inverted.
Now,if you play the song you listen all normal like before.
If you wanna set the mid channel, insert the plugins in GRP M.
If you wanna set the left channel, insert the plugins in GRP L.
With this approach, you can set M and L and listen directly how it sounds in stereo.
Sorry for the long post but it's an italian article and I've synthetized it for you (hope you undertand something

)
I post the link of the article for the pictures of the various steps in cubase.
Tecniche & trucchi per la produzione audio