Supply teachers must become more responsible for their own training as schools become more reliant on them, say experts.
Temporary teaching staff have no training rights and some fear they are falling behind as the profession becomes increasingly research-led.
For many, the flexible hours of supply work fit in with childcare, self- employment or retirement.
But as teachers struggle to find permanent jobs in Wales, more are forced into supply work and risk becoming out of touch quickly.
The General Teaching Council for Wales says 12 per cent of staff are registered as supply teachers, compared with 7 per cent in England. But temporary staff have limited opportunities for continuing professional development.
Unlike with permanent staff, local authorities and schools are not obliged to provide training for supply staff, although some agencies offer courses.
Supply teachers can apply to the GTCW for continuing professional development money as long as they have worked for at least 20 days during the previous year. However, the council has already run out of cash for this academic year.
In 2006, research by the Department for Children, Schools and Families in London found that only 34 per cent of supply teachers in England had any continuing professional development during the previous year. No similar research has been conducted in Wales.
David Evans, secretary of NUT Cymru, said lack of professional development was a major concern for its supply teacher members. "The benefits of training are not only for the teacher but for schools and authorities. It improves the entire workforce," he said.
Ben Edwards, who has been a supply teacher for five years and last year won the New Directions Inspirational Teacher Award for best supply teacher, said they needed to be more proactive. "I was lucky enough to work with some amazing teachers and picked up a lot from talking to them," he said. "I ended up training on the job."
Regular supply teachers may also be invited to take part in schools' in- service training.
Fiona Lewis, a former supply teacher who is now manager the Cardiff office of the Teaching Personnel agency, said schools like continuity. Good teachers who know the policies and layout of the place tend to get asked back.
"We do see supply as a stepping stone - as a way of getting a permanent job," she said. "I often tell teachers to use supply as a networking process. It can even be a chance for teachers who are bad at interviews but great in the classroom to prove themselves. The school gets to see them in their true colours."
But for those on supply who move on to a permanent job via an agency, there is often a "temp to perm" fee which can vary widely.
Mr Edwards initially took supply teaching work out of necessity, but says it now fits in well with running his own business.
"You really can make a difference," he said. "I was inspired by supply teachers at school who brought themselves into the lesson with music or stories. They were the ones you remembered."
Mr Edwards said it pays to be enthusiastic and advises "getting into pupils' good books".
"The thing that always gets back to senior management is what the pupils have said about you - you have to get them on your side in the first five minutes of the lesson."
Be flexible: If you're offered a role in an area you do not specialise in, don't automatically say "No". Schools need people who make themselves useful - and you might even enjoy it.
Be ready for work: Get up early and into the mood for a working day. Some supply teachers get dressed in work clothes each morning ready for the phone to ring.
Get a car: It can be expensive, but transport is a major issue if you're on supply placements. If agencies know you can be in a rural school in half an hour, you will be the first person they call.
Always arrive on time: Know where you're going and leave plenty of time to get there. If you arrive late even once, it can damage your reputation.
Learn pupils' names: Can be tricky but useful for controlling poor behaviour. Send some paper around the class for pupils to write their names on and you have a ready-made seating plan.
Finish any set work: Don't ignore lists left by the class teacher, but don't be afraid to add your own creative touches.
Carry a box of tricks: This is particularly useful for primary teachers when no work has been set. Tried-and-tested worksheets or props for a variety of year groups can be a lifesaver.
Be sociable: Making the first move can be daunting, but get your face known around the school and it could give you the edge in future interviews. Or perhaps lead to a permanent job.