You actually did a decent job. Editing and reposting JPEGs is always going to hurt because the JPEG compression on the rest of the photo has already been done once, and any changes you make will only be compressed once on re-save while the rest will have been done twice. The artifacting will be different.
One trick for the dashes you did is to not create them yourself. Copy a horizontal bar from the rest of the text, preferably with a slightly fuzzed selection, and paste it where you want it. A very slight rotation will help since most people tend to write dashes on signs with a slight incline (direction depending on whether you're a righty or a lefty), but look to the rest of the text for clues. Then don't make the lines exactly the same length, since most people won't write them that way (look at the top and bottom of the "I"). Also notice that their horizontals are thicker and longer, so try to immitate that.