Yes it is important to keep the icing thin enough but not soupy. It is important to use your whole arm - not just your hand to write. Also, keep the tip close to the cake - almost but not quite touching the icing. Make the letters tall but thin/narrow usually. That can change depending on what look you are going for. Can't say it often enough, practice, practice, Practice, *PRACTICE*
There are presses and letter cutters one can purchace to help also.
Another practice thing is to make fondant, colorflow &/or gumpaste plaques in different shapes to practice on. The ones that turn out good can them be used on cakes. It could become you 'signature' style that writting plaques are used
One other thing that can help is to lay a piece of thread or string on the cake to help keep your writing straight.