Good advice already.
One thing to avoid is to send a blanket preapproval letter with some figure in it--such as "Buyers are preapproved up to $400,000." Suppose you're negotiating on a property. The seller's asking $400,000 and you want to offer $350,000. Well, that preapproval at $400,000 is going to signal to the sellers that you can afford to pay the full asking price. The sellers are going to be less willing to sell it for much less.
Instead, as Sally suggests, have the letter state that you're preapproved for whatever figure you're offering . . . say $350,000. That gives the seller assurance that you ARE qualified for what you're proposing, but doesn't give away what you can go up to.
An alternative used by some investors is to have multiple prequalification letters, each with a different amount--such as $350,000, $375,000, and $400,000. That way, they've got a letter that matches closely what they're offering, without revealing what they can go up to.
Hope that helps.