It depends on what you are predicting.
The reason for the prediction is to show how you knew the outcome of a given test before the person knows it herself
If you are doing card predictions I would not recommend having the card turned over upside down in the pack after they select it (presummably this is what you asked about). Rather use an ID if you want something smooth
Mentalism is all about performance and very little about effect. Even if you only do two 'tests' and the rest of the time is just spent discussing matters of the mind, you would have done alot! In this field more is less
Think about what it would be like for the spectator, if you have a sealed envelope which upon being opened tells them exactly what they have just said to 2 seconds ago, what they will be wearing and even the date and time it is said!
Just think about what must be going through that spectators mind!
This is obviously a more elaborate test but you get the idea. Predictions can be powerful in the right hands.
If you are serious about mentalism, get a hold of Annemans Practical Mental Magic and Tony Corinda`s Thirteen steps to Mentalism.
Otherwise, just have fun with the whole mental magic angle
Be careful, some people may ask you if its real...