I've been offering layaway since I started my shop and its been extremely successful!
Here's how I handle layaways
www.etsy.com/shop/crochetgal/policy
Basic terms are 50% down - nonrefundable to hold the item. The balance must be paid off within one month (unless prior arrangements have been made) or the entire payment will be forfeited. I prefer to set it up with no more than three additional payments but this is negotiable. Once the item is completely paid for, it will be shipped. There is a 5% additional charge for layaways. Should you wish to use the layaway plan, contact me before making the purchase so that layaway payment dates can be arranged. The minimum purchase amount on a layaway is $100.