Yes, you can buy a top up card at a store. If you pay sales tax in your area it will be $25 plus tax.
You can also buy top up "cards" over the web. Many of us buy our top up "cards" (actually they just give you a pin code) through
http://www.callingmart.com. If you use coupon code
ca3p-1207 it will bring the price of the $25.00 card down to $24.01 and callingmart doesn't collect sales tax. You will get your top up code right away and save a buck.
Once you have your top up code, access the virginmobileusa.com site and hit the "top up" link in the upper right hand corner of the screen. Enter your phone number and the top up number from the card you bought, either in a store or online. You can do this anytime, even before your account renews.
(You can add money to your account directly from this VM top up page, but then it's $25 plus tax.)
Some people report that VM magically sees that there is money sitting in the account, deducts it on their bill date and keeps their service going automatically. Others, including me, have never had this happen.
On the day my account renews, I have to sign in to virginmobileusa.com and hit the "Restart my plan month" link on the right. Don't do this in advance if you still have minutes remaining or you will lose the minutes and it will change your renewal date to whatever date you do this on.
Some have reported that hitting the "Activate" shortcut in the applications menu on the phone will also do the same thing as hitting the "restart my plan month" on the VM site. I haven't tried this.
If all else fails, you can call VM customer service and pay over the phone ($25 plus tax) and/or ask them to restart your plan month.
This is all kind of a minor pain, but for $25 a month, I deal with it.
If you do happen to burn through all your minutes before your month is up, you can just top up and restart early. You'll have 300 new minutes, but your new bill date will be whatever date you restart on.
You can set up automatic payment by having a credit card on file, but I haven't tried that. I believe that this will cause the account to both auto top-up and auto restart the plan month, but I don't know that for a fact.