I find Paypal works very well for me. It allows me to invoice my clients with a proper invoice that includes my own logo, address and other details. It also allows me to invoice clients in their own currency. Then once the client gets the invoice it is a simple process of a few clicks for them to pay me.
Also I have copies of the invoices and if a client forgets to pay the invoice with a few clicks I can send a reminder to them. Simple and fast