I know there are some teachers who feel strongly that students should not have tapes on their instruments, but I like to use tapes so that students can visually see the relationship between fingers and tapes provide a great starting point when teaching position. For example, I always set up my violin and viola players to keep their thumb on or a little behind the 1st tape. Cello and bass players can use the tapes as a reference when placing their thumb on the neck between the 1st and 2nd tapes.
I've tried few different things as tapes over the years. I used to always buy plain white name-tag sticker labels and I would cut them into strips to use as tapes. This works pretty well, but the adhesive is so sticky that the tapes are hard to remove and these tapes can slide around a bit.
My son's Suzuki violin teacher taught me about car pin-striping tape. These come in a few different colors and they work perfectly as fingerboard tapes. They are just the right thickness at 1/8" and are easily found at any auto supply store for around $2.99 a package or on amazon.com.
My beginning orchestra classes are quite huge so it can take some time to get tapes on all the instruments. To help the process go faster, I have beginners leave their instruments on one side of the room during the first week of school and I have students from my advanced classes place the tapes approximately where they should be without completely sticking the tapes all the way down and around the fingerboard. Then, I am able to quickly check and move tapes to the right spot and stick them down.