Learn to easily your positions right away without having to search for them. The best way to learn to find a position is to have a guideline. Playing a position with your hand between the scroll and body of the instrument makes it difficult to insure you are right until you actually hear the notes and can readjust. Having a guideline will give you confidence even before playing your first note. Practice scales in all positions to reinforce the feeling of being in the correct position.
Begin these shifts with slow slides and have patience. Once you feel comfortable with the positions begin to shift quicker with less slide. Every time you find the correct note try to take a snapshot of the feeling and save it in your brain for future use. Practice reaching up and grabbing the correct note immediately without a slide as well. The more often you can begin in the desired position without fault the more confident you will become starting without test notes.
Lastly, memorize your fingering patterns in all the separate positions for reading purposes. Practice easy works that you know well in different positions regardless of which position works the best, for position practice purposes.