Sorry your having so much trouble but I really stink at it too. But I have been able to find sponge type letting (Michaels) in the kids crafts isle that help put an imprint on the cake for me and I fill that with usually a #5 tip and it has helped, still not perfect but better. Hope this can help you.
For straight lines I'll often lay my viva across my cake and use the edge of it as a line to follow. I've also learned I right more in a straight line if I turn my cake at a angle from me. Actually if I was standing looking at the cake I would the cake in a 1/4th of a turn (counter clockwise) and right with the cake like that.
I stink at this too...I have taken to cutting out letters in fondant and applying to cake! I have 2 sizes of letters! I will write when I have to, but I don't like it! I will have to try the viva trick with the turning of the cake. Thanks for that tip.
My Mom is a sign painter and recently taught me to start at the end of the word.... sounds weird right. But than you know where it will end and it seems to make things fall into place better for me!
What I do is writing the text on the computer with a font I like, with the appropriate size and after that print it on paper. Then I just put the paper on the fondant or the royal icing (if it is not to wet but not dry neither) and gently push over the letters with the tip of a skewer or the handle of a brush. When I take the paper out I have an imprint to guide me for the freehand writing.
I type it up in the font and size that I want, then flip the text backwards using PhotoShop (you could really just turn the paper over once it's printed out so that the text is backwards). I place a sheet of wax paper over my reversed image (think FBCT) and pipe over the letters. I pop it in the freezer for about 10 minutes. Once it's set, I remove it and place it on my cake, icing side down, to make an imprint. I remove the wax paper then I pipe over the imprint.
It's a lot of steps, but I'm always so nervous to write on cakes because I so new at this. I hope I'm able to just pipe away one day!
It took me about two years of solid practice to finally become comfortable with it. I can do it now without blinking an eye. I also turn the cake slightly when I do it. I am a visual person so it helps that I can see it in my head as I am putting it onto the cake.
I have written large scripture passages, think about 3/4 of a 1/2 sheet cake in small writing, on cakes for church things (confirmation versus etc...). As stupid as this is going to sound, it sucked until it didn't. (That deserves a big "DUH" right?). I would stay up late at night after my kids went to bed and just practice writing on wax paper. I would scrape off the buttercream and keep practicing until I couldn't stand it anymore. Eventually it just got easier. I am not sure if that was due to the practice, my determination, or a combination thereof.
All I can say is if you are serious and keep practicing it will get easier. 
I was taught to turn my cakes at a angle then I always start with the largest word first so i can center everything else. Like happy birthday Nikki!
I would write birthday first then write happy above it and Nikki underneath.
and of course practice. I used to write on parchment paper for hours. When I first started working in a bakery I had to learn to write on cakes even though I did not decorate cakes.
Get a book on calligraphy. These techniques are perfect for writing with frosting. Every line is a down stroke. Moving up usually causes the frosting line to break.
When I have to do large writing or various sizes, I use red thread as my line. I mark the edges of the cake with a ruler and a small knife; only a nick is needed. The measurements also include the lines that are the spaces between the written lines. I then stretch the red thread across the cake and attach gently at the marks. After writing the message then I pull straight up on the thread to remove it. Any letters that have dropped across the thread will drop back into place. The border covers the measurement marks.
__________________________________________________
__________________________________________________
__________________________________________________
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This sounds more complicated than it is.
Michele
My Mom is a sign painter and recently taught me to start at the end of the word.... sounds weird right. But than you know where it will end and it seems to make things fall into place better for me!
I used to be a sign painter too!
Starting at the end would only work if you're printing, not for piping script handwriting (unless you're a machine!).
also move your whole arm, not just your wrist like you do when writing with a pen/pencil. Whole fluid movement. Let gravity do the work .... allow the icing to fall from the tip onto the cake.
That said .... I LOVE the impression letters and use those 99% of the time! I have word/letter presses in a couple of different size for cursive and for regular printing.
I print on card stock and then make pin pricks; lay that on the crusted buttercream cake and press lightly. It leaves a nice impression. To get it straight, I shine a beam from my husband's lazer level. I really enjoy doing the writing part. It is sometimes scary when using very dark colors for the writing on very light colored cakes. Getting the consistency of the icing right for the writing also helps it flow without breaking and I use a small damp brush to make little points lie flat. Hope that helps. Remember that Practice makes Progress = not Perfection. 
For the cake in my photos I brought up a large font on my laptop screen to look at, then used a toothpick to mark on the cake where the edges of the letters would go, then used the marks as a guide when piping the letters.
I am new to cakes but I have strong piping skills from doing cookies a long time so this is one of the few things I am confident in. It takes a LOT of practice and the icing cannot be too thick or it won't flow well when piping. It is so much easier to pipe with icing that has a little bit of shortening in it compared to all butter.
Hi Lorabell,
Here's another idea for you (and the only way I can pipe with any control at all): add of few drops of water into mixed SugarVeil, and fill a cartridge of the Icing Dispenser with it. Place a copy of the message/letters under greased parchment, trace using the Icing Dispenser, and when set, peel and place the flexible message onto your cake. There's lots of room here for fixing any mistakes, you can even snip a letter out and replace it with another if you like. Here's a photo of the Icing Dispenser: http://www.sugarveil.com/icing_dispenser/index.htm - you just use it in your hand just like a pen.
You can also build up monograms like I did in the cake that's below:
@Unlimited.. very true, I use this when painting the words on fondant... not piping... a duh moment for me.
Check out the forum I started on this topic, sharing my secrets for writing on cakes.
http://cakecentral.com/cake-decorating-ftopict-666968-pipe.html+font+perfectly
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