I am just curious and thought I would throw this out to all of you.
If you had to choose just ONE thing that you couldn't live without in regards to your cake decorating, what would it be?
And, if you had to give ONE piece of advice or 'words of wisdom' about decorating, what would it be?
Good questions!
But wow...only ONE? Yikes! First thing that comes to mind is all the piping bags and tips. I mean really you can't do much of anything without those. Secondly for me, my biggest help is my dual round/rectangle turntable.
My piece of advice is to read as much as you can here at Cake Central on all the boards for tons and tons of helpful hints! I've learned more here than anyone could have ever taught me.
KimAZ
I couldn't live without my Kenwood mixer or my reversible turntable for icing cakes. I see Kim AZ mentioned the same one, it is priceless.
Words of wisdom? Don't procrastinate, it will make you a nervous wreck if you leave everything till the last minute. When baking set out all of your ingredients side by side, put each one away after you use it. That way you know you haven't forgotten any ingredients in the cake and the kitchen won't be such a disaster when you are finished.
Disposable piping bags! I don't like the vinyl ones, I don't think they're flexible enough and I HATE washing them. I haven't figured out how to deal with parchment triangles yet.
For advice...try anything! When you see a technique you like, try it. That's the only way you can develop your personal style.
I could not live without my oven. Okay, really and seriously, I could not live without all of my bowls. Between some of the recipes requiring three bowls to make the batter, or coloring frostings 4 different shades of pink, I need all of my bowls.
As for words of wisdom...I'm passing on what I heard in a class once...
(okay, it was Collette Peters)...
"It's only cake." Meaning, in the end, if you get too serious about it, you will lose the joy of making it.
I would have to say my KA mixer. My husband tried to "fix" my first one when it wasn't running right, well it never made that noise again! in fact it never made ANY noise again! Needless to say he ran right out and got me a new one! Well I was kinda glad he did ruin the first one cuz I got the color I wanted & it had more watts! 
Advise Hmmm..... Never, ever,if your on a time schedule, go to this site!!!
It will suck you in !! Just kidding , I couldn't live without this site. It has taught me so much & given me so many ideas!
At the risk of sounding hokey, I think I could not live without CC! I often think of my wilton classes as the equivalent of learning the alphabet. However, it is Cake Central that taught me how to write poetry.
My best advice is to not accept/reject a recipe simply because it received a good/bad review in a thread. I have found that some recipes that are well-loved by other CC'ers are not for me. And sometimes ones that are not as well-loved have become my favorite. And a lot of times I agree with everyone else! So try it out for yourself and develop your own compliation of recipes that work for you.
Other than the things you have to have (bags, tips, spatulas) I couldn't live without my KopyCake machine. I use it for almost every cake I do, simply because there is writing on every cake, and my freehand writing skills leave something to be desired! I've seen too many beautiful cakes ruined by sloppy writing to chance it.
I love LOVE my kitchenaid. Couldn't live without it.
Rachel
Aside from the obvious KA (two, actually) I couldn't do cakes without that giant icing/basketweave tip, never remember the number, #781?
Advice? Clean as you go...it makes it much more enjoyable. I love to have a fresh clean area without a bunch of icing-covered, crumb-covered utensils and bowls crowding around the sink; that stresses me out. So while cakes bake, I clean pans and mixers. While the icing whips, I clean counters and sugar syrup pans and measuring cups. While cakes chill with crumb coating, I clean mixer parts and utensils.
So when it's time to pull them out to decorate, voila, clean counters, and I can just enjoy it! 
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