I use paypal and Square (
www.squareup.com)., which is a free application with a free credit card reader. The card reader, which plugs into the audio jack on your phone, has a slot that lets you swipe a card. It's base on the transaction, rather than a monthly service fee, so saves me a ton!
For hourly clients, (billed in 15 minute increments) I get an up-front, non-refundable deposit from new clients that is applied to their first month's bill. i get the full month up front for the occasional monthly 'chunk o' time' client.