Purchase a foam paint roller at your local hardware store. I purchased mine in the paint section at Walmart.
I always like to have a crumb coat on my cakes. After I complete the crumb coat I put the cake in the fridge. Then after it has "crusted" I take it out and put another layer of frosting on. I smooth this layer of frosting the best that I can using a spatula. Then the cake goes back into the fridge. After this layer of frosting has "crusted" I take it back out of the frige. I use the roller just like in painting to roll over the cake. This will help smooth out the lines and bumps. Some lines may take more pressure than others.
This is a picture of the foam roller.
This is a picture of the cake before I used the roller
This is a picture of he cake after using the roller