When icing my cookies I have trouble getting really straight lines. I look at other peoples cookies and their lines are perfect. I use Antonia's Royal Icing. Any tips would be greatly appreciated.
First tip is the one you expected --- PRACTICE!!
While you're practicing, keep these things in mind...
Your icing should be thin enough so it doesn't curl or break when it comes out of the tip. If it's too thick, thin it with a little water.
Hold the tip a little forward of the direction you're piping and let the icing 'fall' onto the cookie.
If you're using a small tip (1 or 2, for example), don't try to go fast. Only a little bit of icing can come out of a tip that small, so don't try to force it. If you squeeze too hard, the icing will curl and if you move your hand too fast, the line will break.