As becky has mentioned, we introduced the online booking system for new starters a while ago and its very successful.
If you were to tackle it from another angle it would be to offer the *first lesson free* which is like a consultation in which builds trust and attracts more students, you would then at the end of the lesson get them to pay for the next lesson and then always be one week in front, this way if they stop lessons you are covered etc, I would do this if you have slots you need to fill but if you are full vet them on the phone and if in doubt don’t book them in without some form of security
the block booking of ten lessons with ten percent discount can work out the same as first lesson free I would make sure they know to book within 28 days etc if its a gift voucher:)