So i have been using Wilton's buttercream recipe for a while. But when I ice the cake it is always bumpy, has holes and I can never have it look smooth. I even sieve the powdered sugar. can anyone help?

I use Indydebi's buttercream recipe and the Melvira method. They work great together. http://cakecentral.com/articles/126/quick-easy-smooth-icing-using-a-roller-melvira-method

I use the buttercream dream recipe from here. have you tried using a Viva paper towel to smooth? I can't believe how great that works. I also found this tutorial helpful:

uk2usa, you will LOVE Sugarshack's instructional DVD, 'Perfecting the Art of Buttercream.' It's worth every dime for someone just starting out (and even after that! ). You (we) might as well learn how to do it right early-on so you don't get into bad habits.
She is Sugarshack here on CC; you can pm her.

Yep, I would have to agree. Sharon Zambitos Perfect Buttercream video will improve anyones skills. She also has a video clip on youtube showing how she makes her buttercream. Used it several times and I have the video too.
Also, if you can get high ratio shortening use that in place of crisco or if unavailable use the store brand without the trans fats removed. It makes a better more stable icing.