Daybeds are beds with a twin-sized (sometimes smaller) mattress and low rails on three sides. During the day, the bed can be used as a sofa; at night, it quickly converts to a twin bed. The rails of these beds are often very uncomfortable, however, and careful use of pillows can make them both more comfortable and more couch-like. Pillows can make daybeds more comfortable and cushy, as well as add to your decor style through careful coordination.
Choose a style. When using pillows to make a daybed into a couch, it is important to carefully consider the decor in the room and choose pillows that will blend with that scheme. You want the daybed-turned-couch to blend into the room, not stand out -- which will only draw attention to the fact that it is not, in fact, a couch. In a romantic, neutral-themed room, you can choose fluffy pillows in pale florals. In a high-use family room, choose big, low-maintenance pillows in dark colours and washable fabrics. In an eclectic studio apartment, heap the daybed with pillows in interesting shapes and bright colours.
Create structure and make it comfortable. While the primary role of pillows on a daybed is to make it more comfortable, they are also important in disguising the bed as a couch. Begin by lining the back and sides of the daybed with large, firm pillows in colours that coordinate with the mattress covering; this will make it look more like a couch. Consider recovering old sofa-back cushions in coordinating fabric, then lining the daybed with them. Or custom-cut a thick piece of foam to fit the length of the daybed, cover it in coordinating fabric, and use this pillow as the base for your daybed.
Make it comfortable. Once the base is set, accent the daybed with smaller, comfier pillows in interesting colours, to make the daybed's rails more comfortable and less rigid. Avoid bed pillows, when possible, as this will detract from your attempts to make the daybed look like a couch. Choose square or bolster pillows in interesting fabrics, just like you would use throw pillows on an actual couch.
Keep it neat. While you don't want to be too fussy, especially in rooms that get a lot of use, regularly tidying up your daybed will keep it looking more like a couch. When it gets a lot of use, the pillows may fall over, ending up arranged more like a heap of pillows on a bed than displaying the structured look of a couch. Take the time to regularly straighten your base pillows and fluff and arrange your throw pillows. Depending on the style of your daybed and how often you actually need to use it as a bed, consider semi-permanently attaching the cushions to the back of the bed, using Velcro or attaching ties to the pillows.