Laegwen made a great point about straining your icing, and some wonderful cookier put a how-to video up youtube. I use meringue powder, so I add water to it and let it absorb while I sift my sugar. Then I run the meringue mixture through the seive to catch any clumps before mixing. I use a silicone or rubber spatula to scrape the sides often, and sometimes even spray mist on the sides to keep them wet, because crust forming is the bane of my existence!
I always move the icing to a clean bowl, and lay plastic wrap on the surface of the icing right up to the edge, then use a tight lid or press n' seal to seal the entire bowl as well. When I take some out and color it, I do the same thing in a smaller bowl, using the mister to thin the consistency because it keeps the sides of the bowl wet and crust-free. Get the consistency perfect for what you are piping, and don't let the clogs terrorize you! Have you seen the basic piping and paper cone videos Julia has up on youtube and her blog? She has tons of awesome advice for piping control and proper positioning.
If you don't use them already, consider switching to parchment paper cones. Making and using them seemed really intimidating at first, but after you make just a few, you see how easy they are to control, to make, and to clean up. They fit the hand better, clog less, and are biodegradable. If one does clog, you can just snip off the end and squeeze the remaining icing into a fresh cone. It simplifies everything, and is a lot less expensive than buying extra tips.
If you are getting frustrated by things like tips always clogging, it makes decorating less fun, and can make you tense. Tension is the enemy here, and if you are worried about getting those lines *just right* it's even worse. You want your back, shoulders, arms, wrists, and hands loose and steady. Shake out your arms often to increase blood flow. Remember not to hold your breath when you concentrate (like I do!), lol.
When you practice a little before piping on cookies, your brain makes a hand-eye connection that you need to do your best. Art teachers in drawing classes will often have students spend ten minutes drawing nothing but lines, circles, and simple shapes before starting, because it helps you loosen up and make this connection. If you do this for a few minutes every day, it makes a huge difference. It may feel like wasted time, but it helps so much!
Put on your favorite music, label some sheets of parchment, and practice for a little bit three times a day for several days. Then go back and see how you did. When I tried it, I stumbled across the fact that caffeine was making my hands shake, and that I am not as steady first thing in the morning or if I am hungry. I made straighter lines holding the tip higher from the surface, going slower, and with my chair height adjusted. The biggest suprise was that when I sang along with music (breathing, lol) instead of focusing intensely, I improved immensely. Literally, the harder I tried to be perfect, the worse I did!
You may have completely different quirks, but knowing them makes all the difference. Remember that many of the cookiers posting here are professionals or have been practicing for years, and some have all the best tools and supplies. They may make several dozen of the same cookie design, and only post a photo of the best one. Plus, they made just as many frustrating mistakes in the beginning. Luckily, most of them are generous and want to help you improve. Comparing yourself to the very best while you are still learning can be disheartening.
Don't be too critical of yourself and stay positive! Remember to have fun! You will get better sooner than you think.