I do not do sketches for cakes under 100 servings, I don't sketch buttercream cakes unless there is some super special detail to them, and if I need a sketch, I use photoshop to make a composite from real photos after they have signed a contract and paid my retainer. My days of loosing orders due to my chicken scratch or not having the absolute perfect shade of pencil on hand is over.
Yes, I have lost commissions because I refuse to sketch at the consultation. But chances are pretty great I wouldn't book those people anyway. I always say (at least it's my sales pitch), "I'm a baker first, a sugar artist second. People that can sketch your dream cake after meeting with you for 30 minutes must be great fine artists - but it says nothing about their ability to replicate that sketch in sugar and have it taste good. I prefer a more organic approach and get to know you and your event details a lot better before I design your perfect cake - and this can take several weeks after your paid booking".
It helps to have a big portfolio to show examples of your work. If you don't have this, I suggest getting some dummy cakes and start adding to it. The more real life examples you can point to, the less you'll have to prove your vision with a pen and paper.