Bit Rusty, need a little macro help.

[Gaffy]

OK, I was a bit off. You need

/castrandom Power Word:Shield, 
 /cast Flash Heal

Make sure you put the comma after PWS. That way when PWS is on CD, it will automatically choose the " " after the comma which does nothing and goes to the next line in the macro.
The downside to this method is that sometimes it will choose the blank instead of the spell and move to the next line. I guess you could minimize this by using

/castrandom Power Word:Shield, Power Word:Shield, Power Word:Shield, Power Word:Shield,
 /cast Flash Heal

up to the 255 char limit.