Provide the details

When making a less than list offer provide some details for the "why". Your Realtor will provide excellent value in this area.
Discuss in a letter to the other side (listing Realtor/owners) your reason for pricing their home the way you did in your offer; examples of similar homes that recently sold, market conditions you as the buyer are facing (relative to the market this seller is selling in), concerns that keep you from 'hitting' their list price. Best to start this letter with how much you like the home. It would seem obvious that you like the home enough to make the offer but don't just jump right into criticism of their home before highlighting their work and effort (a mistake Realtors make when starting a negotiation - I am guilty). If you have seen a ton of homes, waited for a long time to pick the right one let the sellers know. End this letter with why you could be a great buyer and great match with your sound qualifications. Your Realtor is perfect for this as they know the market, know how to communicate your message within the current market environment and they want to help you get that home.