Whipped vegetable shortening with powdered sugar. I can make it up a week in advance, probably even longer, and keep it in an airtight container in a cool place. And it has the advantage that it's pure white (if you use imitation vanilla extract, or lemon extract), so you can get vibrant colors on whatever you're decorating.
If you make too much, you can then make flowers and let them dry out, then store those for a year or more.
Here's the recipe I use, that I got from Jane who taught two of the cake decorating classes I took:
4 lb powdered sugar
1 TB cream of tartar
1 1/4 c. of white shortening (crisco ... the sticks are easy to measure)
a pinch of salt
1 TB extract of your choice
3/4 to 1c. of water (maybe more depending on humidity)
Put all ingredients except the water into a mixer. Add 3/4 c. water, and slowly add more to make the consistancy of your choice. Mix on low speed 'til mixed, then on #6 or 8 for 6 to 7 minutes. Consistency should be "creamy" -- like cream cheese when soft. Stiff icing should be creamy also.