It’s principally a problem between the US and other countries. The JAA has massively assisted contract work within member states and most other countries just require a JAR licensed pilot to sit a air law exam and perhaps a brief flight test. Contract work is usually done by a mixture of word of mouth (for regular contract pilots) or through the agencies that advertise in the back of Flight International. Most contract pilots are rated on at least a couple of different types and cycle between these on different contracts to keep plenty of cards up there sleeves. For example they may do 6 months on a 737 followed by 12 months on an A320 before another contract on the 737. Generally so long as you have flown a type in the last 5 years it’s not too difficult to get back working on it.