I've spent most of my career telling people I can do things and then spend the time learning how. I consider it the "cost of education". Over time I've gotten quite an education. I am at the point in my career however that I'm not interested in learning certain things and will tell a client "sorry I don't do that type of site or feature". But since I've built up quite a clientelle, I have the financial freedom to say that.

Another good approach is to say "I'm not sure if I can do that, let me research it and get back to you". Then you have covered your butt!