Hi,
If your wellies are natural rubber, then wash the dirt off, dry naturally and put a silicone spray on them.
Also store them in a place with an even temperature which is out of direct sunlight.
As the silicone puts a coating on the rubber they should be easier to clean, and you should only need to apply once a month.
I always treat my leather boots the same way, keep them clean, but apply polish instead of silicone. The oils keep the leather soft, and also helps to keep them waterproof, and easier to clean.
I hope this helps you out.
Same for me, a squirt of silicon spray seems to work a treat!