Hi Dawn
I use a free online programme called
Diino - it allows up to 5GB of free storage and upto 100GB for around US$50 per year.
You can share files/folders etc with clients and only those given access can view/amend files.
I have not tried to access files at the same time as my client but it does allow just the one copy to be saved and not worry about large files to be emailed with the problems that ensue there.
It also has the option to let you/your client make a list of tasks - easy way of ensuring you both know what you are meant to be doing and by when.
From another Dawn!