Take the time to press each seam to the right side as soon as it is sewn. This simple procedure will make block assembly a much smoother and more accurate process. Do not "iron" as you would an item of clothing. "Press" by placing and lifting the iron straight up and down without distorting the small pieces of fabric.
One of my new favorite tools for foundation piecing is the Clover Mini Iron. The nifty iron is just the right size for pressing seam allowances and working in tight spaces. This iron is great for smoothing appliqued bias strips and any other small pieces of fabric.