A full shelf in the greenhouse...

we recycle and repurpose as much as we can and here is one of our adopted ways.... straight out of Linda Woodrow's book, The Permaculture Home Garden. We scrounge polystyrene lidded boxes from the rubbish pile at the Supermarket - they have seafood delivered to them in them - sometimes they are a bit smelly! Then cut them in half to make two trays. Have to be a bit careful with the half that comprises the lid as it's base, though mostly this is fine. From the recycling section at the dump we take back as many square, 2-litre, plastic milk bottles and cut off the top and the bottoms, leaving about 15cm+ open-ended rectangle. Then fill the poly trays with the cut down milk bottles - sometimes I find a row of square, 1-litre bottles is needed if the shape of the tray doesn't fit neatly. We fill these with our potting mix and transplant our seedlings in one per milk bottle as soon as they have their first pair of true leaves - sometimes the seedling is only a few cms tall. We leave them to grow here until they are about 15cms tall, so they are strong and sturdy by the time they are ready for the garden outside. We feed them with worm wees whilst in the greenhouse. At the moment I'm using a commercial mix, after running out of an easy supply of 2 of my ingredients for home-made potting mix....will get back to that soon....so the worm wees is a great boost for the seedlings.
The punchline for this system is, of course, that one simply makes the hole in the garden bed where the seedling is to be planted and gently tips the whole milk container into it, until the soils are level, tuck in with extra compost/mulch and shimmy up the milk bottle to create a perfect micro-climate/protection barrier around the new plant until it has settled in (2-3 days). Easy.