I find that practicing in silence to get the handling smooth works well until until I reach the point where I don't need to think about what comes next.
Then, I stop thinking (as much as I can)about the handling and perform for an imaginary audience where I react to their comments, they answer questions and select cards ect. Until this runs smooth and any extra verbiage is pruned away.
Now, it is ready for rehearsals and there is no other way than to shanghai someone, anyone, to run through with a warm body to observe and see how they react to the effect. This is the place where, ultimately, the decision to add to roster or to drop it comes in.
I usually run through 2 or 3 new effects that i run through twice and then stop. Brainless repetition does no good and I think is detrimental to the process.