I've heard people read it this way: "If your delight is in the Lord, he will 'give you' - i.e. place in your heart - the right desires you ought to have." He will determine the desires of your heart. Instead of the 'prosperity' reading, we could call this the 'sanctification' or 'holiness' reading. It's about our giving God the blank cheque. Although this sounds like good theology and pious devotion, exegetically (i.e. in regard to interpretation) it sounds a little fishy, does it not? "He will determine your desires" seems like a rather awkward way of reading "he will give you the desires of your heart." Is there a third way?
One of the basic and most important principles of reading the Bible, or anything else for that matter, is to read in context. In this case, lifting one isolated verse or clause out of a psalm can lead you down an exegetical dark alley and get you into a whole lot of theological trouble. I wonder how many ex-Christians have pulverized their fragile faith over a mishandled scripture like this one. READ THE WHOLE PSALM! What is it about? What is happening in this piece of sacred prayer-literature-turned-liturgy? "Fret not yourself because of evildoers" (v. 1) ... "the man who carries out evil devices" (v. 7) ... "The wicked plots against the righteous" (v. 12) ... David (we'll assume) has enemies. Someone is giving him grief. "The wicked draw their sword and bend their bows to bring down the poor and needy, to slay those whose way is upright" (v. 14) ... This is now getting serious! David is facing a death threat ... again. When was the last time you faced real enemies ... the kind who are actually trying to kill you? No, we're not talking about that obnoxious neighbour, that gossipping relative, or that bullying boss. An enemy who is trying to put an end to your life? Most of us live in a very different world from David's, don't we?
As he prays, what does David desire? What is he expecting from God? "Trust in him, and he will act. He will bring forth your righteousness as the light, and your justice as the noonday" (vv. 5-6). Ah .... justice! In the face of an enemy who wants to wrongfully (and quite literally) destroy him, David wants things set RIGHT. Look at verses 9 and 11 ... he wants a peaceful place to live (in those days, having a piece of land, a place to sleep and a field to grow crops, is a basic necessity). Now read verse 25 - he wants BREAD! I may be tempted to ask for barbecued steak or ribs. But the fact that David is expecting to receive bread - in case you didn't know, bread is used for keeping people from starving to death - helps to put things into perspective! Here's what David desires and is hoping for ... this is what his faith is all about: "The wicked watches for the righteous and seeks to put him to death. The LORD will not abandon him to his power or let him be condemned when he is brought to trial" (vv. 32-33). This is about salvation in a very Old Testament sense: deliverance and peace.
Now go back and read verse 4 ... and keep reading till verse 6: "Delight yourself in the LORD, and he will give you the desires of your heart. Commit your way to the LORD; trust in him, and he will act. He will bring forth your righteousness as the light, and your justice as the noonday." Does it now make easy, obvious sense? Yahweh is a God who looks after his covenant people. He meet their basic needs (also compare Ps 145:16). He grants shalom. Rather than the 'prosperity' reading or the 'holiness' reading, we could call this the 'deliverance' reading, which fits well not only with this psalm, but with the general flow of the Psalms and the Old Testament as a whole.
Now do you want see something really cool? (Ok ... I'm a geek!) Look up Matthew 6:33 (or better yet, read the whole chapter), and compare with Psalm 37:4 (and the whole Psalm around it). Look up Matthew 5:5 ... a paraphrase of Psalm 37:11. Give it some thought. Do you think Jesus had Psalm 37 (among others) in mind as he preached the Sermon on the Mount? What was that sermon all about? Do you think Matthew 6:25-34 is about prosperity? A blank cheque? What about the passage before it, Matthew 6:19-24? Is it not just the opposite? "You cannot serve God and money"! Shall we not, rather, be people who seek first God's kingdom, confident that our daily needs for food and clothing will be met along the way? Shall we not be grateful, and even embarrassed, that God has so extravagantly prospered us, that we can afford to spend two dollars on a Tim Hortons coffee or twenty on a restaurant meal? (or two pounds on Costa!) Matthew 6:34 sums it up well: "Therefore do not be anxious about tomorrow ..."