You can try blocking but if it's stockinette, it just wants
to curl and it will have its way. This is a very familiar topic. Lots of discussion here.
and some ideas here
in part one of a series on fixing curling. In some cases it's helped by a deep border of garter or seed st but that may not be possible for the small items you're knitting. Maybe lining the item?
Hope you can find a solution!