i made a dairy free frosting with margarine (2 sticks) and crisco (1 stick) and a 2 lb. bag of powdered sugar. I was altering my usual dairy buttercream recipe. I substituted rice milk for the regular milk to get it nice and smooth and fluffy. I usually add 5-6 Tbs. of milk, but after adding only 3 Tbs. of rice milk with the margarine and crisco (also had a few drops of imitation butter flavoring and 3 teaspoons of vanilla) it is very soft.

It is already VERY sweet, so I don't know if adding more sugar is the answer. How can I stiffen it up for piping? Flour?

Thanks. The cake is for tonight! Yikes!

You do not want to add flour or cornstarch (ppl have done it lol) Add more powdered sugar, you can add some salt to cut the sweetness down a little (from what I hear)

I have added some cornstarch in the past. If it's not much it won't be tasted. Adding more sugar is really the best and a generous dash of salt helps.
In the future when you want to pipe cut down on the amount of fat used in making the icing..

The problem was from the margarine (make sure you read the label to be sure it is dairy-free).

Margarine has more water than butter. In future, it's best to use shortening only.

If the nut is OK then try using coconut milk from a can, it makes amazing icing as well as cake.

So I ended up adding more ps and it turned out okay. I did serve it cold from the fridge because it seemed like the frosting was getting too soft at room temperature.

The margarine was definitely dairy free. The next cake that I am making is using a mostly chocolate frosting with just a few piped details...for that I may use pure shortening to keep it stiffer. I like the idea of trying canned coconut milk...it does seem thicker although it will impart a bit of a coconut flavor (which may not be to the palate of kids).

The frosting tasted good and got good reviews from adults and kids alike.

Thanks for the tips.

AI have cooked and baked with rice milk before & often I find it far to liquidly to be used as an actual milk. If you're afraid that the taste of coconut will mess up the flavor, I'd suggest almond milk. It's definitely not as watery as rice milk & it doesn't have that strong of a flavor profile. You will have to be careful of people's allergies to almonds though. Just some food for thought ;)

Soft icing: Add more icing sugar to make it stiff.  

Stiff icing: Add teaspoon of milk and mix until you reach the desired consistency. :)