One option is Payments Pro if you plan to store clients credit card details. Payments Pro does have a monthly fee ($30.00) plus the standard transaction processing fees. Pro does require a higher level of programming skills.
Else, assuming you have a US PayPal Business Account, using Payments Standard you can setup either a subscription or recurring payment button for you clients. You can do this by creating an item button for a subscription which can be included on a web page or you can send an email text link version to your clients.
