‘Reasons are the pillars of the fabric of a sermon; but similitudes are the windows which give the best lights’ (Thomas Fuller).

Spurgeon expands the latter thought in this quotation, by suggesting that illustrations, like windows:-

1.  should let in light: they should clarify and explain our meaning.  Joshua Shute wrote: ‘That sermon has most learning in it that has most plainness.  Hence it is that a great scholar was wont to say, “Lord, give me learning enough, that I may preach plain enough.”‘
2.  are pleasurable and interesting.
3.  give opportunity for introducing ornament.  ‘Meretricious ornament we deprecate, but an appropriate beauty of speech we cultivate.’
4.  introduce a breath of fresh air into the proceedings.
5.  should not be too numerous.  ‘They are not the strength of a sermon any more than a windwo is the strength of a house.’
6.  should really cast light on the subject in hand.  ‘Blind windows are fit emblems of ilustrations which illustrate nothing.’
7.  should not be too prominent.  ‘Our figures are meant not so much to be seen as to be seen through.’
8.  are best when they are natural, and form part of the whole plan.  ‘No illustrations are half so telling as those which are taken from familiar objects.’
9.  should never be crude or dirty.  ‘A house is dishonoured by having dirty, cobwebbed windows.’
10. should not be broken or cracked: beware of fractured metaphors.

(Based on C.H.Spurgeon, Lectures to my Students)