Lots of people overbeat cream cheese frosting thinking it'll get fluffier, but it just turns loose and runny, especially after sitting out. We learned that the hard way at Zinfandel Grille. Mixing slow is what keeps it thick and smooth. Also, frosting a warm cake? That just melts it. Be patient and wait for the cake to cool completely so your frosting actually holds its shape.
Don't use cold cream cheese straight from the fridge. It won't mix smoothly and you'll get lumps that are impossible to fix. We started letting ingredients sit out at Dirty Dough Cookies about six months ago, and that grainy texture in our frosting is just gone. Also, add the powdered sugar slowly. Dumping it in all at once makes a runny, overly sweet frosting. Take your time for a sturdier, better taste.
At ERI Grants, we joke that cream cheese frosting is just like grant compliance. Easy on paper but easy to mess up when you try to practice. The most common mistake of home cooks is using too warm cream cheese, which causes the frosting to become loose and soupy all too quickly. Another big one is having the mixing process getting out of hand. People think that as smooth as possible is better, too much mixing breaks up structure and is runny. Another culprit is using low fat cream cheese. It just does not hold up. Finally, the sugar may be added too quickly to throw off the balance. Slow and steady mixing and patience is all that is required. When you respect the process, then the results are smooth stable and reliable.