I am in the midst of working on a piece outlining some guidelines for seeing Christ in the Old Testament without allegorizing or damaging the Old Testament texts themselves, and in my prep, I found these good rules of thumb from Charles Spurgeon. In his Lectures to My Students, the chapter titled "On Spiritualizing" isn't about Christ in the Old Testament per se, but it does offer some great advice on how to (what Spurgeon calls) "spiritualise" a text without doing so excessively and inappropriately. Here are his rules:
1. Do not violently strain a text by illegitimately spiritualising.
Under this point, Spurgeon tells a story about a preacher who turned a proverb about gluttony into a warning against listening to bad preaching. This is an example of spiritualising that doesn't find deeper or Christological truth beneath or beyond a text but patently denies the plain sense of the text to find a "higher" meaning. Spurgeon advises that good "spiritualising" never ignores the primary meaning of a text or violates good ol' fashioned common sense.
2. Never spiritualise upon indelicate subjects.
This is a peculiar point, but apparently there were men in Spurgeon's day who enjoyed turning some texts into references to things best not discussed in public. He is aiming at those who find sexual innuendo in everything. (I had an English professor who did that quite a bit, with every story, and I have no doubt if he would have ever found the Bible worth a second of his time, he would have done the same with it.) Spurgeon cites how some treat Solomon's Song as an example. Surely nobody does that these days, right? ;-)
3. Never spiritualise for the sake of showing what an uncommonly clever fellow you are.
Some men make stuff up, and in doing so only mean to show they are good at making stuff up. Their motives for finding a spiritual meaning is to pridefully reveal their own ingenuity.
4. Never pervert Scripture.
Connected to the first rule, this one is not just about denying the plain meaning of a text, but actually cutting against it. Spurgeon mentions a fellow who says the seventh commandment is a word of obedience from God the Father to God the Son, telling him not to covet the devil's wife (ie. the non-elect). That's not only stupid, not only denying the plain meaning, it perverts the truth. It shows heterodoxy. Good spiritualizing is always in keeping with good theology.
5. In no case allow your audience to forget that the narratives which you spiritualise our facts.
In other words, do not obscure the history. Spurgeon cautions here that in finding "deeper truths" to the Old Testament stories, we don't turn them into myths or parables. (This will be an important aspect to the Christ in the Old Testament article I am working on. It is crucial. The aspect, not my article. ;-)