I pretty much have crappy handwriting.
Some people make a fondant plaque and write on that so if they mess up it isn't on the cake itself. Others use fondant and tappets or other shapers. Try different fonts (check the gallery for other peoples writing) and see of one works better for you than others. Some people stagger the letters up and down so it doesn't have to be lined up. Try printing vs writing. The biggest thing is to practice. Take a bag of icing and practice on the bottom and sides of a pan, scrape it back in the bag and reuse it, over and over again. Practice a little every day. Another thing is to not have your icing so stiff that it doesn't hold together in a stream or is too hard to push out with good control.
It helps to watch, if you are a visual learner.
I made a video
v v v v v v v v v v v v v v
There is really only one way to master a craft: practice.
I'm new to cake decorating and I'm about to start my 3rd Wilton class. I have the same issue so far and I thought about tracing the letters with piping gel on parchment paper and then transfer to the cake and ice over it. I've never tried this technique just an idea.
Look up the pattern locator button at wilton.com and search for "script". There are MANY pages of messages that can be used for daily practise.
Prepare a batch of soft-ish icing. Fill up a small parchment cone every day and write the whole thing out onto printed paper sheets with the script patterns. Start with printing: http://www.wilton.com/technique/Print-Writing and then follow the link for script.
Within a week, you should be able to see the difference.
Practice with a tube of cheap toothpaste
Its about the same consistency and it can be scraped off your piping practice board and used over and over again - it won't crust - block your tubes or cause damage to your equipment
Its what I use for students ALL the time
Thank you everyone for your great suggestions!