I personally like the Inland InstaHeat because its ceramic heating element helps it hold a very even temperature, but there are many good ones to choose from. Don't waste your money on an 80 Watt iron, but spend the little bit extra and get a 100 Watt model. You'll never get a smooth bead with a dirty iron. One is especially important if you use different types of solder. Don't worry over any given bead too long at one time.
Most flux breaks down quickly and begins to hinder more than it helps.
Once it does, the solder oxidizes such that it will never be bright and shiny. You also run the risk of cracking your glass due to overheating. Let a problem area alone to cool down completely, clean it, reflux it lightly, and tickle it again. Don't even try to finish solder an area that's dirty with, for example, foil adhesive. Clean it and start again.