I'm just a hobby baker myself and I also have a little trouble with writing. What I have found is that if I hold the tip closer to the cake the writing is a lot neater. Some people also recommend that you write with a toothpick and then go over it in icing.
I think that making a line with a little ruler or making the letters first with a toothpick is a good idea so that the writing isn't crooked. Also, planning where the letters will be so that your words don't go right off the cake. When piping let the icing "fall" onto the cake. Keep in mind... when writing with a pen, your fingers do the work, when writing with icing, your wrist/arm does the work. And practice first on some wax or parchment paper. Hope that helps you out a bit. 
My instructor taught me using the toothpick. I pick the font I want, print what I want to write on the cake out on computer paper, then set it on my cake and trace the letters with a toothpick. Then I go over that with my thinned icing. Works great.
I have a lot better luck if I don't try to write in cursive. Also when I pipe out the lettering instead of trying to make it perfectly straight and even I make it uneven or a little off on purpose. I can usually do it pretty easy freestyle that way and it kind of makes it look fun and not so formal. Hard to explain but I have a few cakes in my gallery you can see what I mean. Oh...and practice, practice, practice!
For printing, consistency doesn't much matter.
For cursive handwriting, thin your icing with more liquid or piping gel. You want it soupy enough to flow. Don't "draw" it on, pipe just a little above the cake to let it drop and write fast.
It's helpful to watch others write. Have you seen my video in the link below?
I use a laser leveler to make straight lines on my cake, for writing.
It shoots a line of light over the cake. When I'm done, there are no lines to see.
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