Standard advice:
1) Drink lots of fluids
2) Get liver function and CBC tests done every 4-6 weeks to check for potentially fatal (but reversable if caught in time) side effects
3) Take something for liver protection (milk thistle, SLO, etc)
4) Be prepared for a rough period. Not everyone gets one, but lots of people do.
5) Plan to stay on it for at least a year
From personal experience:
My daughter had no problems at all with Valcyte. I, OTOH, had a rough period of about 8 weeks which resembled an IRIS reaction.
http://en.wikipedia.org/wiki/Immune_reconstitution_inflammatory_syndrome. It's tempting to give up during the bad period but if your tests are coming back okay, you might want to tough it out. I had a big improvement right after my IRIS-like reaction.
IMO, you'll get the most out of Valcyte if you also do a MAJOR rest program. My doc told me to rest. When I didn't, I plateaued. When I did, I got better faster. For me (and daughter) more rest meant more improvement.
My daughter took Valcyte for 18 months and went into remission. She's on Valtrex indefinitely to keep the herpesviruses suppressed now that they're latent again. I was on Valcyte for 2 years and am also currently on Valtrex. I am not in remission, but am greatly improved -- bedbound to not-quite-housebound and with many fewer symptoms.
Good luck! Keep us posted on how you're doing.