In order to do that you need an appropriate ember, which are items you find hidden throughout the game. Each blacksmith can only work with specific embers, and you must go find them and give them to the appropriate blacksmith before you can modify your weapons using them. Andre, the blacksmith you have been using deals with standard, which is normal +5 to +15, divine, occult, and raw. The Giant blacksmith, who is in Anor Londo, deals with Lightning, Crystal, and special boss weapons. Rickert, who is in New Londo, deals in magic, and enchanted. And Vamos, who is in the Catacombs, deals in Fire, and Chaos upgrades.
Most of them can be good, but it all has to do with how you want to level up your character. Do you want to continue increasing your strength and dexterity stats?