Dahlias are not frost hardy, which means unless your garden is in a sheltered spot the tubers cannot say in the ground over winter as they will freeze. It is necessary to lift the tubers in the Autumn usually done when the foliage is blackened by the first autumn frosts.
To do this, dig up and lift the plant, and then cut off the top growth, so you have just stem and tuber. Take care when digging up the tuber not to put the fork through the tuber or damage it as a cut or bruise can lead to rot when storing the tuber overwinter. Throughly clean off all clinging soil, and this is usually best done by hand to ensure the tuber is clean for storage. Place the Dahlia in frost free place to dry out moisture retained in the tuber which could take up to 3-4 weeks.
Once the tuber is fully dry, place the tuber in a box and cover the stem with vermiculite/sand/or dry compost to keep dry and frost free over the winter. It's a good idea to label them. If you are lucky, the Dahlia tubers will survive without mildew, saving you the cost of buying them again next spring. Check regularly and if any are showing signs of rotting, discard. It is not unusual to lose some tubers over the winter.
To make lifting Dahlias easier, you can grow them in containers and when planting, sink the container into the border. At the end of the season, rather than dig up the individual tubers, the entire pot can be lifted in Autumn and then the tubers dried and stored.
Dahlias are definitely a red wheel barrow plant. I have seen Dahlias described as "easy to grow" but in my book they are high maintenance. Of course, it is often the case of "right plant right place", and in the more sheltered, warmer places in the UK Dahlia are easy to grow. Visiting the Cotswolds is such an example; marvellous Dahlias in abundance, flowering beautifully, and no need to lift in the Autumn.