There’s nothing worse than a dirty iron. It never fails that it will leave a spot in the middle of your white fabric. I’m a fan of Hot Iron Cleaner.
The package suggests putting 2″ of the cleaner on your hot iron, but I’ve found using a smaller amount works better. It sort of reacts like butter in a hot pan. I find that using smaller amounts (about 1/2″) and repeating the process works best. I use fabric scrap to clean. It keeps my fingers from burning and is trash anyway!