Overwintering plants is a good way of keeping tender, and slightly less than hardy plants, from year to year. Which plants you can overwinter depends really on available frost free space.
If overwintering plants under glass the main problem is likely to be botrytis, grey mould particularly in a green house. In the winter plants need air and the greenhouse needs air to circulate to prevent mould. To protect against cold it is tempting to wrap up the plants and block off all the draughts which will create just the still damp conditions mould enjoys.
To reduce the risk of mould, space plants well apart, avoid their leaves touching and prune if necessary. Water sparingly, open vents and doors as much as possible and raise plants off the ground on a trestle, see image below. At the first signs of mould, which forms on leaves as grey fuzzy powder which collapse to the touch remove the plant and any leaves to stop it spreading.