What I did after I found out the work my client needed done and what his monthly budget was (this was a great way to discuss a retainer). Ask you client how much they have budgeted for a VA?? My client said they had $xx.xx in the budget. I let him know that since this was going to be an on going working relationship I did required a retainer of $XX.XX (the same amount he said was in his buget). I explained that if by chance he needed work that went over this amount I would let him know a head of time. At the end of the month I provide a detailed invoice to my client and deduct the amount he owes from the retainer and then he provides the additional funds to bring the retainer back to $xx.xx. I hope that isn't to confusing.
I think the biggest thing is to ask as if you already know your client will pay it. Over the years I have learned by assuming you will get what you are asking for human nature just follows. If they by chance don't explain you require this for the first 6 months while you are establishing a business relationship. Personally, if this is your policy I would stand by it...
Good Luck