There are two categories of pressing aids- liquid or spray formulas you apply to the fabric, and objects used to shape or protect the fabric. The latter category includes pressing hams, which enable you to press a curved seam, and the oblong version which is great for pressing sleeves. A press cloth placed between the iron and the fabric prevents scorching or the creation of a shiny surface. A thick towel or needle board is used when pressing velvet- the fuzzy side faces the towel.
My two favorite liquid pressing aids are Faultless Premium Starch and a simple vinegar and water solution (50/50) in a little spray bottle. They both add body and crispness to your work. Starch does stain some fabrics, so always test it on a scrap first. I have never had a problem with the vinegar solution, but I would recommend checking that on a scrap first as well. The vinegar solution really is amazing and has become an indispensable aid in my sewing room.
Next tip- freezer paper in the sewing room